Evolution, Societal Sexism, and Universal Average Sex Differences in Cognition and Behavior
Abstract and Keywords
During the past century, social scientists have documented many cross-cultural sex differences in personality and behavior, quite a few of which now appear to be found in all human societies. However, contrary to most scientists’ expectations, these so-called universal sex differences have been shown to be more pronounced in Western industrial societies than in most non-Western developing societies. This chapter briefly reviews the evidence bearing on these findings and offers a biologically based theory that could help shed light on why cross-cultural sex differences exist. The following hypothesis is offered: The expression of many genes influencing sexually dimorphic traits is more likely among descendants of couples who are least closely related to one another. If so, societies in which out-marriage is normative (i.e., Western industrial countries) will exhibit a stronger expression of genes for sexually dimorphic traits compared to societies in which consanguineal marriages are common.
This chapter focuses on better understanding numerous average sex differences in cognition and behavior. Because the phrase is used frequently, it is henceforth abbreviated with the acronym ASDCBs. This abbreviation refers to any and all average sex differences in how humans think and behave. For ASDCBs that appear to exist in all societies and time frames, the term universal ASDCBs is used.
There are five parts to this chapter. Part 1 documents that numerous universal ASDCBs now appear to exist. Part 2 reviews evidence of how ASDCBs seems to vary in strength over time and across countries. In Part 3, findings from cross-cultural research on the nature of ASDCBs are described. Part 4 describes three theories for explaining ASDCBs, one being strictly environmental and two being of an evolutionary/biological nature. Part 5 explores how well the three theories explain what current evidence suggests about ASDCBs, as revealed in Parts 1–3.
In today’s fast-paced communications world, some readers may simply be interested in this chapter’s bottom line. For them, a brief conclusion section provides an overview of all five parts of the entire chapter.
Part 1: Universal ASDCBs
Social scientists have been searching for possible universal ASDCBs for many years, some with expectations that few, if any, would be found (Kessler & McKenna, 1978; (p. 498) Mead, 1963). Doubts about the existence of universal ASDCBs began to change in the 1970s with the publication of a book by Maccoby and Jacklin (1974). It summarized findings from several thousand studies published up to the early 1970s. Their review led them to identify four ASDCBs that seemed to be present across all cultures: (a) superior verbal ability in females, (b) greater visual–spatial ability in males, (c) better mathematical ability in males, and (d) more physical aggression in males.
Two decades later, a meta-analysis of thousands of additional studies was published by Feingold (1994). It brought him to confirm conclusions reached by Maccoby and Jacklin (1974) and then to add five more universal ASDCBs. Females, he said, were more anxious, friendly/gregarious, trusting of others, and tender-minded, whereas males were more assertive.
A decade and a half after Feingold’s (1994) meta-analysis was published, eight colleagues and I published a book in which findings from more than 18,000 studies pertaining to sex differences were summarized (Ellis et al., 2008). Citations to the studies were organized into hundreds of different tables, each one pertaining to a separate possible ASDCB. Many of the tables had to do with strictly biological traits and included information on nonhumans, neither of which are of concern here. The majority of tables, however, actually pertained to some aspect of human cognition or behavior. From these tables, evidence of 65 universal ASDCBs was obtained. Methodologically, we designated an ASDCB as being apparently universal if (a) at least 10 relevant studies had been conducted and (b) each study without exception reported the same sex difference to exist to a statistically significant degree. For those interested in the details, each ASDCB is described in Ellis (2011a). To save space, here I merely provide a brief sketch of the nature of these 65 ASDCBs under seven subject headings:
1. Stratification and work: Twelve behavioral traits were identified having to do with social stratification or work. They indicate that males work longer hours outside the home and are more likely to be employed in a variety of “male-typical occupations” such as jobs of a supervisory, scientific, and engineering nature. Females, on the other hand, when employed outside the home, are more likely to work in people-oriented and caregiving occupations.
2. Drug consuming and illegal behavior: Central to the five traits under this category is that males consume more alcohol and engage in more criminal behavior than do females.
3. Social and play behavior: The general pattern seen in this category of 12 traits indicates that females are more cooperative and helpful to others throughout life and even in their childhood play activities. Males, on the other hand, tend to be more competitive and more prone to interact with members of the opposite sex in explicitly sexual ways.
4. Personality and general behavior: Seven personality and general behavior traits were identified. They boiled down to males throughout life being more inclined to explore their environments, to take greater physical risks, and to behave in hostile/aggressive ways toward one another. Females were found to be friendlier. In all the countries sampled, females also expressed greater concern about being overweight.
(p. 499) 5. Attitudes and preferences: Twelve universal sex differences in attitudes and preferences were found. Males express greater interest in physical science and technology, and they want to watch and participate in sports more often than do females. Females have a greater preference for marriage partners who are taller and wealthier than themselves, whereas males want mates who are shorter and younger than themselves. Females have greater interest in school, whereas males have more interest in sex.
6. Mental health: Twelve universal sex differences involved mental health issues, broadly defined. In this regard, alcoholism, learning disabilities, attention deficit disorder with hyperactivity, psychoticism, and autism are all more common in males. The cognitive/mental health traits that females exhibit more often are anorexia, bulimia, and panic attacks. Females are also more likely than males to blame themselves for any shortcomings, and they ruminate over unpleasant social experiences more.
7. Emotions and perceptions: The last category of universal sex differences involves females perceiving greater hazards in their environment, reporting greater feelings of stress, and crying more as adults. Males report feeling bored more.
Regarding emotions, readers might suspect that there are other universal tendencies, such as the tendency for women to be more depressed than men. There is in fact considerable evidence supporting this particular conclusion (Hopcroft & Bradley, 2007). However, a few exceptions exist (Ellis et al., 2008, p. 373). To give one of the most recent examples, data obtained from China revealed that equal proportions of men and women self-reported feeling depressed (Hopcroft & McLaughlin, 2012, p. 510).
Overall, it now appears safe to say that many universal ASDCBs exist. If one uses the criteria we set for confident conclusions in this regard, the number stands at 65. But if somewhat more liberal criteria are used, the number could be in the hundreds.
Of course, the search for universal ASDCBs is not over. Findings from hundreds of new studies of sex differences are published every year. No matter what criteria one sets, it is possible that (a) more universal ASDCBs will be located in the future and (b) exceptions to those already identified may be eventually unearthed. With these provisos in mind, some rather surprising evidence pertaining to ASDCBs is briefly described next. This evidence has to do with the types of cultures in which one finds the greatest degree of sex differences in cognition and behavior.
Part 2: ASDCBs Over Time and Across Cultures
Societal efforts have been made in Western cultures to treat the sexes more equitably for well over a century. One of the earliest landmarks in this regard occurred in 1893 when New Zealand became the first country in the world to grant women the right to vote, (p. 500) a right now afforded to all women living in democratic countries (Ramirez, Soysal, & Shanahan, 1997).
In many Western countries, particularly the United States, laws now guarantee women equal access to higher education (Klein et al., 2014), to sporting activities (Hargreaves, 2002; Milner & Braddock, 2016), and to employment opportunities (Rossilli, 2000). Women’s rights activists continue to work toward even greater equality, but undisputed progress has been made. For example, in Western societies, most adult women are now in the paid labor force, a dramatic increase over the past century (Durand, 2015). Even more dramatic changes have occurred in terms of higher education. At the start of the 20th century, roughly 90% of college graduates in industrialized countries were men; by the early 1990s, well over half of all college degrees were being awarded to women (Averett & Burton, 1996; Buchmann & DiPrete, 2006; Cho, 2007).
Given these trends, it is worthwhile asking if they have been paralleled by any changes in ASDCBs. For example, Have people’s attitudes toward sex equality shifted? Have sex stereotypes changed? Or, have people’s self-perceptions of themselves in masculine/feminine terms diminished? Although much of the relevant data are limited to the United States, the results are quite interesting. The following is a summary:
US attitudes regarding sex equality: One study sought to determine if people in the United States are becoming more sympathetic to the idea that men and women should be treated more equally. To address this question, it compared responses to various questions about sex equality that were first administered in the early 1960s and then again in the mid-1990s (Thornton & Young-DeMarco, 2001). Findings indicated that substantial shifts have occurred. In particular, over the three decades involved, greater proportions of people of both sexes believed that men and women should be treated equally in interpersonal relations and in employment opportunities. From this investigation, it appears safe to infer that US attitudes regarding the desirability of treating the sexes equally have become more favorable in recent decades.
US sex stereotypes: If attitudes toward treating males and females more equally have become more favorable, one might be led to believe that sex stereotypes have diminished. Sex stereotypes, of course, refer to the extent to which people believe that males and females behave differently regarding a wide range of traits. To address this question, one study compared sex stereotypes expressed by US respondents first in 1974 to similar respondents in 1997 nearly a quarter of a century later (Lueptow, Garovich-Szabo, & Lueptow, 2001). In both cases, respondents were given a list of behavior traits and asked to rate them as being either more typical of males (masculine) or of females (feminine, or not different regarding sex). Overall, this study concluded that nearly all of the stereotypes have remained virtually unchanged. The only exception involved a slight decrease in the extent to which females were stereotyped as exhibiting certain feminine traits, but there were no significant changes in the extent to which males were stereotyped as possessing masculine traits (see also Lueptow, 2005).
(p. 501) US masculinity–femininity self-perceptions: Another investigation sought to assess trends in personality sex differences by meta-analyzing findings from studies that had all used a popular measure of masculinity–femininity, known as the Bem Sex Role Inventory (Twenge, 1997). All of the studies included in the meta-analysis were conducted in the United States predominantly among college students from the mid-1970s through the early 1990s. Analyses revealed no significant changes in the degree to which males considered themselves to be masculine or feminine over a wide variety of interests and preferences. Females had not changed their self-ratings in terms of possessing masculine interests and preferences, but women sampled in the 1990s did express somewhat more feminine interests and preferences than did women sampled in the 1970s. Thus, although serious validity issues have been raised regarding the Bem scale (Hoffman & Borders, 2001), Twenge’s meta-analysis suggests that despite all the changes that have occurred both culturally and legislatively in terms of more equal treatment of men and women, the extent to which US college students perceive themselves regarding masculine or feminine traits has changed very little.
Part 3: ASDCBs Across Cultures
E-mail communications and computerized data management have made it possible for researchers throughout the world to conduct large-scale cross-cultural studies. One such study involved assembling responses from prior studies of more than 23,000 respondents residing on four different continents (i.e., Africa, Asia, Europe, and North America). The goal was to search for varied sex differences in personality traits (Costa, Terracciano, & McCrae, 2001). In particular, the meta-study sought to determine if people living in Western countries (i.e., those in Europe and North America) exhibited stronger or weaker sex differences in personality traits compared to people from predominantly non-Western countries. Results revealed that across all four continents, similar sex differences existed in nearly all of the personality traits measured. However, to the amazement of the researchers, the degree of sex differences in personality traits was more pronounced in the Western countries than in the predominantly non-Western countries (p. 322). Thus, despite all the social and legislative efforts made to promote sex equality in Western countries, sex differences in personality traits were greater in the Western countries than in countries in which few such efforts have yet to be undertaken.
Another research team investigated personality traits using measures from the Big Five Personality Inventory from more than 17,000 respondents in 55 countries (Schmitt, Realo, Voracek, & Allik, 2008). The team’s work revealed that countries in which people’s health, lifespan, years of education, and wealth were the highest (i.e., predominantly Western industrial societies) were the countries with the greatest degree of sex differences in personality traits. Figure 23.1 shows how the findings from Schmitt et al.’s (p. 502) study correlate with those from the study by Costa et al. (2001). Note in particular how both studies are in agreement that the greatest sex differences in personality are found predominantly in affluent Western countries, whereas less affluent non-Western countries have the fewest sex differences in personality.
Subsequent work by Schmitt (2015) involved measuring personality traits among respondents drawn from 26 different countries. It further reinforced the conclusion that the most extreme average sex differences were among respondents drawn from the most industrialized and affluent predominantly Western societies. Similar conclusions about affluent Western societies exhibiting the greatest sex differences have been reported not only for personality traits (McCrae & Terracciano, 2005; Schmitt et al., 2016) but also for depression, for which females typically surpass males (Hopcroft & Bradley, 2007; Hopcroft & McLaughlin, 2012).
In the case of academic achievement, most studies have found that females outperform males at least through adolescence (Ellis et al., 2008, pp. 278–279). A recent international study of more than 1 million adolescents was undertaken to determine if there were fewer tendencies for females to outperform males in predominantly Western (p. 503) egalitarian countries as opposed to countries in which equality of the sexes is generally discouraged. It found no significant differences in this regard (Stoet & Geary, 2015).
Overall, although Western attitudes toward sex equality have become more prevalent in recent decades, sex stereotypes and sex differences in ASDCBs (e.g., self-concepts and personality) have changed very little. Even more surprising, when Western industrial societies are compared to non-Western developing countries, sex differences in personality traits appear to be more pronounced in the former than in the latter. What in the world is going on? Perhaps theories of ASDCBs can shed light on these rather curious findings.
Part 4: Theories of ASDCBs
Ideas on why males and females seem to think and behave differently have been around for a long time. Proposals began to solidify enough to be identifiable as scientific theories much more recently. Basically, three theories with distinct properties can be identified.
Social Role Theory
The concept of social roles began to be used by social scientists in the 1940s. It refers to how human behavior often seems to be heavily influenced by the training and expectations one receives from others (e.g., parents, teachers, and other influential persons) within a particular culture as though people’s behavior is being scripted by others (Merton, 1968; Parsons, 1942). This perspective began to take the form of a theory of sex differences in behavior in the 1960s (Rosenberg & Sutton-Smith, 1968; Tulkin, Muller, & Conn, 1969).
Social role theory (also sometimes called sex role theory) argues that all sex differences in behavior are learned through the socialization process. Basically, each individual learns what is culturally expected of men and women, and most then gradually conform to those particular norms depending on their own sex (Bussey & Bandura, 1999; Kessler & McKenna, 1978). To illustrate, one of the first things any expectant parent wants to know about his or her baby is whether it is a boy or a girl. As soon as one learns the answer, from birth onward, boys and girls are treated differently on average, and it is this differential treatment as well as a child’s understanding of how he or she is expected to behave that cause average sex differences in behavior to develop throughout life (Eagly, 2013; Eagly & Wood, 1999; Eagly, Wood, & Diekman, 2000; Ridgeway & Correll, 2004).
Currently, among the most prominent proponents of social role theory are Eagly and Wood (1999; see also Eagly, 2013). They assert that were it not for learning and powerful socialization processes, males and females would be all but identical in their behavior. In their words, “extensive socialization is required to orient boys and girls to function differently” (Wood & Eagly, 2002, p. 705). Another proponent summarized social role theory as follows (Rogers, 2005): “Men are expected to be ‘aggressive’ and unemotional, (p. 504) women to be sensitive, intuitive, etc. From a very early age, they learn what is expected of them in terms of ‘feminine’ or ‘masculine’ personality, and this is heavily reinforced at puberty” (p. 11).
Social role theory reflects what has been termed a “blank slate” perspective because it assumes that males and females would behave the same if it were not for the societies in which they live having different expectations of what is “appropriate” behavior for males and females (Pinker, 2002, pp. 337–371). In other words, social role theorists “expect gender differences in personality to be smaller in cultures with more gender egalitarianism” (Schmitt et al., 2016, p. 1). It is worth keeping in mind that two surveys have both indicated that social role theory is by far the most popular theory for explaining sex differences in behavior, at least among sociologists (Horowitz, Yaworsky, & Kickham, 2014; Sanderson & Ellis, 1992).
Beginning in the 1970s, some social scientists began to move toward studying ASDCBs from an evolutionary perspective (Daly & Wilson, 1978; Simon, 1980). Since then, evolution-based proposals for explaining ASDCBs have expanded a great deal (Buss, 2012; Campbell, 2013; Geary, 2010; Hopcroft, 2016; Lippa, 2010; Mealey, 2000; Schmitt, 2015). Without denying the role of learning or culture, proponents of this perspective emphasize that biological and reproductive factors are even more important.
The main elements in all evolutionary explanations for ASDCBs can be summarized as follows: Biologically, the purpose of life is simply to produce more life, especially life resembling whatever organisms are currently living. In sexually reproducing species, two individuals are required to make new life: one male and one female. For males and females to attract one another, each must exhibit certain traits. In highly evolved species such as humans, the “right” combination of male and female traits for attracting the opposite sex includes behavior as well as physical characteristics.
To provide a simple illustration of the previously discussed line of reasoning, imagine that there are two groups of females in a human population. Group 1 prefers mating with males who control a continual supply of food and other resources and are willing to share them with their mates. Group 2 has no interest in males with resources, preferring instead males who appear to be young and attractive from a health standpoint. Which group of females will pass their genes on at the highest rate? Group 2 would have an advantage in the sense that young and health mates would be more likely to produce healthy offspring. However, Group 1 females would gravitate toward mates with resources to sustain them throughout each pregnancy. Especially given the lengthy gestation periods for each human pregnancy and the dependence that each offspring has on a stable supply of resources, under most circumstances, the females in Group 1 would probably out-reproduce those in Group 2.
Now pose the same question for two groups of males. Again assume that the males in Group 1 prefer females who control and share resources, whereas the males in Group 2 (p. 505) are primarily drawn to females who appear youthful and healthy. Keep in mind that because they do not gestate offspring, males have a much higher reproductive ceiling than do females. So which group of males would most likely have the most surviving offspring? By choosing females who control many resources, the Group 1 males would be drawn to women who are willing to spend considerable time working, which would probably limit the time these women would devote to gestating offspring. On the other hand, the Group 2 males who are most drawn to females who are relatively young and attractive would be able to pass their genes on at considerably higher rates, provided they (the males) are willing to devote time to resource provisioning.
The preceding evolutionary reasoning may seem almost too simple for its implications to be true in reality. However, notice how scenarios match evidence that males emphasize youthfulness and physical attractiveness more than females do when choosing mates, whereas most females are more interested in mating with males with the capacity to make a “decent living” (Conroy-Beam, Buss, Pham, & Shackelford, 2015; Zentner & Mitura, 2012). It is impossible to prove that these sex differences in human mate preferences are evolutionarily based, but the fact that there is virtually no society in which these mating patterns do not exist (Ellis et al., 2008, pp. 441–444; Shackelford, Schmitt, & Buss, 2005) is at least consistent with such an explanation.
Also, notice how the evolutionary arguments just presented can be extended to help explain some other well-documented sex differences:
1. Compared to females, males are more likely to choose jobs based on how much money they can earn (Ellis et al., 2008, pp. 462–463).
2. On average, among full-time workers, males work longer hours (Ellis et al., 2008, p. 782).
3. In every society ever studied, mothers spend more time caring for their children than do fathers (Ellis et al., 2008, pp. 651–652).
The overall point being made is that evolutionary reasoning can provide useful conceptual tools for making sense of quite a few apparently universal ASDCBs. Of course, social role theorists might explain these same sex differences by arguing that each one reflects a “cultural script” of what males and females should do. As discussed further in the following section, the main problem with the “cultural script” hypothesis is that it implies that very few, if any, universal ASDCBs should exist.
Evolutionary Neuroandrogenic Theory
It may be possible to strengthen conventional evolutionary theory, at least in terms of explaining ASDCBs, by combining it with evidence of how genes, hormones, and the brains of males and females differ. Biologically, the only things that survive indefinitely (p. 506) after an organism dies are the genes carried by its descendants. Within a given species, these genes come in two forms, one for males and the other for females. By and large, the collection of genes for males and for females are extremely similar. The main exception involves the so-called sex chromosomes, of which females carry two X chromosomes and males have just one X along with one Y chromosome.
In essence, female mammals (including humans) are the default sex, with males being just a variant on the female sex (Dennis, 2004; Woodson & Gorski, 2000). Genes on the Y chromosome serve to effectively switch the would-be female ovaries into becoming testes instead. Testes are special organs for producing a sex hormone known as testosterone along with other so-called male sex hormones, collectively known as androgens.
Hormones are biochemicals that are produced in one part of the body and then transferred to other parts of the body (usually via the blood system) where they have their primary effects. Although testosterone and other androgens are produced mainly in the male testes, small quantities are also produced in the female ovaries and in the adrenal glands of both sexes. Studies have shown that bodily exposure to androgens has many effects, including the promotion of muscle and bone tissue (Leonard et al., 2010), thereby explaining why males on average are stronger and taller than females (Ellis et al., 2008, pp. 15 and 30). However, it is the effects of androgens on the brain that are of greatest importance regarding ASDCBs.
Research has shown that androgens alter the brain in many ways, including the size of various parts of the brain, the biochemicals being released, and how the brain actually functions (Baron-Cohen, 2004; Kimura, 1992; McHenry, Carrier, Hull, & Kabbaj, 2014). These androgenic modifications occur both prenatally and postpubertally, with the prenatal effects of androgens being the most profound and irreversible (Auyeung, Lombardo, & Baron-Cohen, 2013; Baron-Cohen, 2004). As one would expect, many parts of male and female brains have been shown to differ on average, both structurally (Ellis et al., 2008, pp. 54–78) and functionally (Ellis et al., 2008, pp. 79–87).
To help explain universal ASDCBs, I have proposed a theory that can also be considered merely an extension of Darwinian evolutionary theory. Thus, it is called evolutionary neuroandrogenic (ENA) theory (Ellis, 2011a, 2011b). The theory stipulates that androgens have evolved as the main biochemicals responsible for masculinizing/defeminizing the brain of an otherwise female mammal. In other words, ENA theory asserts that androgens not only masculinized/defeminized the body as a whole (e.g., increased muscularity) but also masculinize/defeminize the brain specifically. Because the brain is the direct controller of both thought and behavior, ENA theory offers the following explanation for universal ASDCBs: By androgenizing the brain, natural selection has tailored male brains to exhibit thoughts and behavior that promote a male reproductive strategy and female brains to exhibit thoughts and behavior conducive to a female reproductive strategy. The nature of both strategies is the types of thinking and behavior that helps each sex pass on genes to future generations.
In more detailed terms, ENA theory asserts that androgens have evolved the ability to not only masculinize/defeminize the body as a whole (e.g., increased muscularity and bone density) but also masculinize/defeminize the brain, thereby impacting thought (p. 507) and behavior. The resulting sex differences in thought and behavior must serve both sexes’ overall reproductive interests. More precisely, as a result of male brains being more heavily androgenized, they mainly produce masculine thoughts and behavior, most of which are part of an overall male reproductive strategy. Female brains, on the other hand, have evolved tendencies to retain feminine thoughts and behavior, which are part of a predominantly female reproductive strategy.
If the above reasoning is correct, one can make the following deductions: Most universal ASDCBs have evolved by natural selection and have been promoted by exposing the brains of males to high (male-typical) levels of androgens. This means that androgens are biochemicals naturally selected for modifying the “normal” female brain into a male brain.
It is worth adding that the nature of the evolved sex differences may not always be immediately apparent. For example, nearly all studies have found males on average to be more accurate than females when throwing objects such as balls or darts at targets (Ellis et al., 2008, p. 240). This ability may contribute little or nothing to male reproductive success in today’s industrial societies. However, in the past, it is likely to have enhanced male hunting ability, which in turn allowed males to provide resources to mates who were gestating and breast-feeding their offspring. Of course, the average number of offspring they successfully rear would be the “reproductive payoff” for the better hunting ability of these males (and for the females who chose these males as mating partners).
ENA theory goes beyond conventional (Darwinian) evolutionary theory in the sense that it specifies how neurohormonal factors has been naturally selected so as to produce ASDCBs. This extra element in turn allows numerous testable hypotheses to be derived from ENA theory about ASDCBs that conventional evolutionary theory does not allow one to deduce.
To illustrate, consider the field of criminology. Many proposals have been made in recent years that evolutionary theory may help explain why males are more criminal than females, especially regarding serious property and violent offenses (Buss, 2012; Daly & Wilson, 1990; Duntley & Shackelford, 2008; Ellis, 2005; Roach & Pease, 2013). Most of these proposals hinge on the fact that males have a much higher reproductive ceiling than do females (i.e., a male can have many more offspring in a lifetime than can a female). Of course, for males to capitalize on their higher reproductive ceiling, they need to have numerous sex partners. But, as already noted, females generally prefer males with resource-procuring abilities. To meet expectations in this regard, males must usually compete with other males. Among the fastest ways to obtain resources and thereby to attract potential mates is for males to engage in thefts, burglaries, robberies, and embezzlements.
Additional criminal methods males can employ to acquire mating opportunities involves the use of physical force against prospective mates—that is, by committing rape or sexual assault (Ellis, 2005). Males can also effectively compete with rival males for resources and mating opportunities by assaulting or even murdering these rivals (Buss, 2012, Daly & Wilson, 1990).
The bottom line is that because males can reproduce much more prolifically than females, many males appear to have evolved several “dirty tricks” (that nearly (p. 508) all governments seek to suppress with the criminal justice system) to help pass their genes on to future generations. Of course, these evolutionary arguments are difficult to directly test. However, because ENA theory links its evolutionary arguments directly with arguments about the effects of androgens on the brain, it provides additional testable hypotheses about sex differences in offending. For example, if ENA theory is true, criminals should have brains that are more highly androgenized. In other words, although almost all males should have higher androgen levels than females, criminal males should have even higher androgen levels than males in general.
Recently, a colleague and I tested this line of reasoning using two separate samples, one from the United States (Hoskin & Ellis, 2015) and the other from both the United States and Malaysia (Ellis & Hoskin, 2015). In both studies, we provided respondents with a checklist of delinquent and criminal acts for them to self-report. To measure brain exposure to prenatal androgens, we used a measure known as the 2D:4D ratio. This measure simply involves the relative length of the second and fourth digits (usually on the right hand). Basically, the longer the fourth digit (ring finger) is compared to the second digit (pointing finger), the greater the exposure to prenatal testosterone and possibly other androgens (Manning, 2009).
Our findings were consistent with what ENA theory predicts: Respondents with the lowest 2D:4D ratios reported greater involvement in delinquency and crime than respondents with the highest ratios. We even tested the hypothesis separately by sex and found the same basic pattern: Both males and females with the lowest 2D:4D ratios self-reported more crime compared to their counterparts with relatively high 2D:4D ratios. This evidence, of course, does not by itself prove ENA theory because the theory basically makes the same predictions for all evolved traits that exhibit average differences between males and females (Ellis, 2011a, 2011b). Therefore, ENA theory provides a conceptual platform for theorizing about all manner of ASDCBs with more ways of being disproven than is true for conventional evolutionary theory.
Part 5: Theoretically Explaining Societal and Temporal Variations in ASDCBs
The last issue to be covered in this chapter involves assessing each of the three theories just identified—social role theory, conventional evolutionary theory, and ENA theory—regarding their abilities to explain findings reviewed in Parts 1–3. In other words, which of these three theories can account for findings about the apparently universal ASDCBs documented so far?
Regarding Part 1, both of the evolutionary theories have an advantage over the social role theory. This is because social role theory assumes that cultural learning is largely responsible for ASDCBs. If this were true, one would not expect to find very many, if (p. 509) any, universal ASDCBs. To give just a few examples, why would alcoholism and interests in engineering be more common in men in all known societies if these sex differences are the result of culturally based learning? Similarly, one should be able to find societies in which boys express greater liking of school and seek to be more cooperative with others compared to females, but as of yet, such sex differences have failed to materialize in any empirical study (Ellis, 2011a).
In the case of the two evolutionary theories, both can explain the findings reviewed in Part 1 by simply noting that evolutionary forces have operated on human populations in essentially the same way for thousands of years. Therefore, if most ASDCBs are either directly or indirectly the result of natural selection, one would expect to find many universal ASDCBs. It is worth mentioning that ENA theory goes on to predict that the vast majority of universal ASDCBs will be associated with differential brain exposure to androgens, an implication that largely remains untested.
In the case of the findings reported in Part 2, the picture is more mixed in terms of judging the merits of the three theories. Recall that the main conclusions drawn from the studies reviewed in Part 2 pertained to trends in the United States. These trends were as follows:
1. Attitudes have become more accepting of equality between the sexes in recent decades.
2. Sex stereotypes have remained more or less stable during the past few decades.
3. Self-perceptions by men and women in terms of their being masculine, feminine, or somewhere in between have changed very little in recent decades.
Because social role theory considers culturally based learning to be responsible for all ASDCBs, it would probably explain attitudes toward sex equality also in terms of cultural learning. Thus, as anti-discrimination laws began to be passes in the United States especially in the 1960s, social role theorists would expect public opinion to shift away from sex discrimination, which it certainly has. Both of the evolutionary theories, however, are essentially silent to the possibility of attitudinal changes regarding sex equality, so they are weaker than social role theory in this regard.
Regarding the apparent stability of sex stereotypes and self-perceptions concerning people’s feelings of masculinity/femininity, both evolutionary theories would probably have an edge over social role theory. This is because they envision most ASDCBs as having evolved over thousands of years, making them unlikely to change significantly over one or two generations.
Part 3 indicated that sex differences in personality traits were more pronounced in most Western industrial societies than in most non-Western developing countries. This poses a serious challenge to all three theories of ASDCBs. Social role theorists would be hard-pressed to explain why societies in which the greatest efforts have been made toward sex equality would end up exhibiting the most sex inequality, at least regarding personality. In the case of the two evolutionary theories, they are largely silent regarding any cross-country comparisons of sex differences in personality.
(p. 510) Because ENA theory has elements beyond what is found in conventional evolutionary theory, it might be possible for researchers to compare citizens from Western and non-Western societies in terms of average androgen levels. If sex differences in androgen levels are higher in countries with the greatest sex differences in personality, it could begin to provide a theoretical basis for explaining Western/non-Western patterns in this regard. Along these lines, a study by Manning, Fink, and Trivers (2014) compared countries in terms of their citizens’ average prenatal androgen exposure and the percentage of elected officials who were females and the percentage of the paid workforce who were females. The study concluded that countries in which female exposure to prenatal testosterone (as indicated by 2D:4D ratios) was high and in which male exposure was low had the highest proportions of females in elective offices and in the labor force. This study seems relevant to ASDCB determination. However, notice that it implies that citizens of Western societies have fewer average sex differences in exposure to androgens than do citizens of non-Western societies. The studies summarized in Part 3 by Costa et al. (2001) and Schmitt et al. (2008), on the other hand, lead one to conclude that Western societies have greater average sex differences in personality than do non-Western societies. These seeming inconsistencies call for more empirical scrutiny.
This chapter was divided into five parts. Part 1 documented growing evidence of numerous universal ASDCBs (average sex differences in cognition and behavior). In particular, work by colleagues and myself led us to tentatively identify 65 such traits (Ellis, 2011a, 2011b; Ellis et al., 2008).
In Part 2, studies pertaining to three questions about ASDCBs were explored. First, have people’s attitudes toward sex equality changed, and if so, in what direction? The answer is that these attitudes have changed. At least in the United States, acceptance of women being educated and allowed to work in jobs alongside men has grown considerably since the 1970s (Thornton & Young-DeMarco, 2001).
Second, have sex stereotypes changed, and if so, how? The answer appears to be that sex stereotypes have changed very little during the past few decades at least in the United States. Specifically, people’s beliefs about how males and females differ in terms of basic interests, personality, and behaviors appear to have remained virtually the same in the 1990s as they were in the 1970s (Lueptow, 2005; Lueptow et al., 2001).
Third, have there been changes in men’s and women’s self-perceptions regarding their masculinity/femininity? In other words, do the sexes today think of themselves as just masculine, feminine, or somewhere in between, as was true in their parent’s generation? The evidence suggests that the answer is that little has changed, with the possible exception of contemporary women having somewhat more feminine interests than those sampled in the 1970s (Twenge, 1997).
(p. 511) The issue addressed in Part 3 had to do with making cross-cultural comparisons regarding sex differences in personality. Surprisingly, the evidence suggests that greater sex differences exist in most Western industrialized societies than in most non-Western developing societies (Costa et al., 2001; Hopcroft & Bradley, 2007; Hopcroft & McLaughlin 2012; Schmitt et al., 2016). This finding seems counterintuitive because considerably more effort has been made in most Western societies to encourage sex equality than has been made in most non-Western societies.
Part 4 described the three theories that have been proposed for explaining ASDCBs. The oldest one to be proposed is social role theory (Eagly & Wood, 1999). This theory asserts that males and females would be virtually identical in how they think and behave if it were not for sex differences in sociocultural training and expectations (Eagly et al., 2000). Surveys among sociologists indicate that it remains the most popular theory among social scientists (Horowitz et al., 2014; Sanderson & Ellis, 1992).
Beginning in approximately the 1970s, Darwin’s theory of evolution began to be specifically applied to the study of human ASDCBs (Hrdy, 1981; Wilson & Daly, 1978). Since then, numerous others have advocated an evolutionary approach to the study of ASDCBs (Archer, 1996; Geary, 2010; Hopcroft, 2016; Mealey, 2000). An evolutionary perspective explains why males and females think and behave differently based on how the sexes differ in their contributions to the reproductive process. In other words, the rate at which humans leave descendants (and therefore their genes) in future generations depends heavily on males and females thinking and behaving differently. Thus, whether operating through differential learning or some biological process, the most reproductively successful males will exhibit thought and behavior patterns than on average differ from those of the most reproductively successful females.
I have proposed a version of evolutionary theory that specifically incorporates brain and hormonal concepts (Ellis, 2006, 2011a, 2011b). This theory, called evolutionary neuroandrogenic (ENA) theory, is specifically designed to explain average sex differences in cognition and behavior. According to this theory, all mammals (including humans) are essentially females. However, a special chromosome has evolved that carries genes for making roughly half of these would-be females into males instead. In essence, these genes operate by causing the would-be female ovaries to develop into testes, special organs for producing large quantities of a masculinizing hormone called testosterone (along with other male hormones, called androgens). Androgens affect thought and behavior by infiltrating the brain both prenatally and following puberty. As a result of greater androgen exposure, males on average end up thinking and behaving in ways that serve their reproductive interests, whereas low androgen exposure in females causes them to think and behave in ways that generally contribute to their reproductive interests.
Finally, Part 5 addresses the question of how well the three theories described in Part 4 can shed light on the evidence summarized in Parts 1–3. Social role theory has difficulty explaining why there are numerous universal ASDCBs as discussed in Part 1. It also predicts that ASDCBs would weaken as societies become more sex egalitarian, which Parts 2 and 3 indicate have not happened.
(p. 512) Because evolutionary theory assumes that most ASDCBs have been naturally (or sexually) selected, it is able to account for why large numbers of ASDCBs exist, as indicated in Part 1. Furthermore, the evidence cited in Part 2 that people’s stereotypes about sex differences and their self-concepts in terms of masculinity/femininity have changed very little in recent decades is also understandable in an evolutionary context. ENA theory, as an extended version of evolutionary theory, goes on to hypothesize that androgenic effects on the brain explain how ASDCBs have evolved. It remains to be seen if most ASDCBs are the result of neuroandrogenic factors, but at least one recent test of this hypothesis regarding sex differences in mate preferences provided moderate support (Ellis & Ratnasingam, 2015).
The evidence that sex differences in personality traits appear to be more pronounced in Western industrial societies than in most non-Western developing societies (Part 4) presents a conundrum for all three theories of ASDCBs. It is especially devastating to social role theory, which is based on the assumption that gender differences would be least prevalent in cultures that foster the greatest degree of “gender equality” (Schmitt et al., 2016). The fact that most sociologists subscribe to this theory more than any other (Horowitz et al., 2014; Sanderson & Ellis, 1992) suggests that a sea change is in the offering for how social scientists think about cognitive and behavioral sex differences.
There seems to be only a few ways to explain why the most sex egalitarian societies have the greatest degree of sex differences in terms of personality traits. One possibility is that more of the genes responsible for sex differences in cognitive and behavioral traits have accumulated in Western populations than in non-Western populations. If so, one would be likely to find that sex differences in physical traits would also be more prevalent in Western populations, a possibility for which I could find no evidence.
Another possibility is that by being more lenient in allowing its citizens to express themselves in sex-related terms, males in Western cultures may end up thinking and behaving in more male-typical ways, and females in more female-typical ways, than is typical of non-Western cultures. If this second line of reasoning is true, it would turn traditional sex-role theorizing on its head. In other words, social scientists who argue that one should not stereotype or try to influence people’s behavior according to “conventional sex roles” because it contributes to sex inequality are mistaken. To the contrary, the effects of culturally prescribed sex roles would actually be inhibiting the expression of sex differences in thought and behavior. Thus, the more equitably males and females are treated by their sociocultural surroundings, the more different men and women will become.
In closing, I offer a proposal for social scientists to consider and test during the ensuing years. It is partly based on evidence that arranged marriages, particularly among close relatives (usually first cousins), are widespread both historically and even today in most developing countries (Desai & Andrist, 2010; Jurdi & Saxena, 2003). Contemporary developed countries appear to be among the only cultures in which arranged consanguineal marriages are rare (Hatfield, Rapson, & Martel, 2007).
I propose that one of the effects of freely marrying individuals from large pools of nonrelatives is that this cultural practice allows the expression of genes for traits to be (p. 513) maximized. This includes genes responsible for sexually dimorphic cognitive and behavioral traits. If so, it is in societies in which out-marrying is most common—that is, primarily Western industrial countries—that one will find the expression of genes for masculine traits in males and feminine traits in females to be the greatest.
Overall, it is clear that research is still needed to identify ASDCBs and to understand the factors responsible for them and for why they appear to be more pronounced in some cultures than in others. Nevertheless, the accumulation of ASDCB research so far is yielding tantalizing surprises that could fundamentally challenge some of sociology’s more cherished assumptions.
The comments provided by Rosemary L. Hopcroft, Anthony Hoskin, Malini Ratnasingam, and David P. Schmitt on drafts of this manuscript are greatly appreciated.
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