The variability hypothesis, also known as the greater male variability hypothesis, is the hypothesis that human males generally display greater variability in traits than human females do.
It has often been discussed in relation to human cognitive ability, where some studies appear to show that males are more likely than females to have either very high or very low IQ test scores. In this context, there is controversy over whether such sex-based differences in the variability of intelligence exist, and if so, whether they are caused by genetic differences, environmental conditioning, or a combination of both.
Early history The notion of greater male variability-at least in respect to physical characteristics—can be traced back to the writings of Charles Darwin. When he expounded his theory of sexual selection in The Descent of Man and Selection in Relation to Sex, Darwin cites some observations made by his contemporaries. For example, he highlights findings from the Novara Expedition of 1861–1867 where "a vast number of measurements of various parts of the body in different races were made, and the men were found in almost every case to present a greater range of variation than the women" (p. 275). To Darwin, the evidence from the medical community at the time, which suggested a greater prevalence of physical abnormalities among men than women, was also indicative of men's greater physical variability.
Although Darwin was curious about sex differences in variability throughout the animal kingdom, variability in humans was not a chief concern of his research. The first scholar to carry out a detailed empirical investigation on the question of human sex differences in variability in both physical and mental faculties, was the sexologist and eugenecist Havelock Ellis. In his 1894 publication Man and Woman: A Study of Human Secondary Sexual Characters, Ellis dedicated an entire chapter to the subject, entitled "The Variational Tendency of Men". In this chapter he posits that "both the physical and mental characters of men show wider limits of variation than do the physical and mental characters of women" (p. 358). Ellis documents several studies that support this assertion (see pp. 360–367), and
"By the 1890s several studies had been conducted to demonstrate that variability was indeed more characteristic of males...The biological evidence overwhelmingly favored males as the more variable sex."
Early controversies in the 20th century The publication of Ellis's Man and Woman led to an intellectual dispute about the variability hypothesis between Ellis and the statistician Karl Pearson, whose critique of Ellis's work was both theoretical and methodological. After Pearson dismissed Ellis's conclusions, he then "presented his own data to show that it was the female who was more variable than the male" Ellis wrote a letter to Pearson thanking him for the criticisms which would allow him to present his arguments "more clearly & precisely than before", but did not yield his position regarding greater male variability.
Support for the greater male variability hypothesis grew during the early part of the 20th century. During this period, the attention of researchers shifted towards studying variability in mental abilities partly due to the advent of standardised mental tests (see the history of the Intelligence quotient), which made it possible to examine intelligence with greater objectivity and precision.
One advocate of greater male variability during this time was the American psychologist Edward Thorndike, one of the leading exponents of mental testing who played an instrumental role in the development of today's Armed Services Vocational Aptitude Battery ASVAB. In his 1906 publication Sex in Education, Thorndike argued that while mean level sex differences in intellectual ability appeared to be negligible, sex differences in variability were clear. Other influential proponents of the hypothesis at this time were psychologists G. Stanley Hall and James McKeen Cattell. Thorndike believed that variability in intelligence could have a biological basis and suggested that this could have important implications for achievement and pedagogy. For example, he postulated that greater male variation could mean "eminence and leadership of the world's affairs of whatever sort will inevitably belong oftener to men." In addition, since the number of women that fall within the extreme top-end of the intelligence distribution would be inherently smaller, he suggested that educational resources should be invested in preparing women for roles and occupations that require only a mediocre level of cognitive ability.
Leta Hollingworth's studies
By examining the case records of 1,000 patients at the Clearing House for Mental Defectives, Leta Hollingworth determined that, although men outnumbered women in the clearing house, the ratio of men to women decreased with age. Hollingworth explained this to be the result of men facing greater societal expectations than women. Consequently, deficiencies in men were often detected at an earlier age, while similar deficiencies in women might not be detected because less was expected of them. Therefore, deficiencies in women would be required to be more pronounced than those in men in order to be detected at similar ages.
Hollingworth also attacked the variability hypothesis theoretically, criticizing the underlying logic of the hypothesis. Hollingworth argued that the variability hypothesis was flawed because: (1) it had not been empirically established that men were more anatomically variable than women, (2) even if greater anatomical variability in men were established this would not necessarily mean that men were also more variable in mental traits, (3) even if it were established that men were more variable in mental traits this would not automatically mean that men were innately more variable, (4) variability is not significant in and of itself, but rather depends on what the variability consists of, and (5) that any possible differences in variability between men and women must also be understood with reference to the fact that women lack the opportunity to achieve eminence because of their prescribed societal and cultural roles at the time. Additionally, the argument that great variability automatically meant greater range was criticized by Hollingworth.
In an attempt to examine the validity of the variability hypothesis, while avoiding intervening social and cultural factors, Hollingworth gathered data on birth weight and length of 1,000 male and 1,000 female newborns. This research found virtually no difference in the variability of male and female infants birth weight, and it was concluded that if variability favored any sex, it was the female sex. The general consensus today is that boys have a higher birth weight than girls, and are more responsive to their mother's diet while in the womb, thus causing an even greater variation if nutritional needs are met.Eriksson, J. G., Kajantie, E., Osmond, C., Thornburg, K., & Barker, D. J. (2010). Boys live dangerously in the womb. American journal of human biology : the official journal of the Human Biology Council, 22(3), 330–335. https://doi.org/10.1002/ajhb.20995
Modern studies The 21st century has witnessed a resurgence of research on gender differences in variability, with most of the emphasis on humans. The results vary based on the type of problem, but some recent studies have found that the variability hypothesis is true for parts of IQ tests, with more men falling at the extremes of the distribution. Publications differ as to the extent and distribution of male variability, including on whether variability can be shown across various cultural and social factors.
2004 twin chromosome study A 2004 study attempted to test for the effects of the averaging of the x-chromosomes through a sample of about 1000 female twins and 1000 male twins at each age from 2 to 4. They hypothesized that MZF (monozygotic female) twins due to x inactivation would have lower correlations with their co-twin than males and the opposite would be true for DZF (dizygotic female) twins due to females sharing an x chromosome from their father. They found statistically significant results for:
Prosocial behavior at age 2 for MZF (lower) and at age 3 for DZF twins (higher)
Peer problems for MZF twins (data only available for age 4)
Verbal ability at age 3 for MZF (lower) and DZF (higher) twins
Anxiety at age 4 for MZF (higher). The inconsistent result with the hypothesis likely means there are no X-linked loci for anxiety.
No statistically significant results were found for MZ or DZ twins for: Hyperactivity, conduct problems, total behavioural problems, nonverbal ability, or g.
2007 metanalysis A 2007 meta-analysis found that males are more variable on most measures of quantitative and visuospatial ability, making no conclusions of its causation.
2008 analyses A 2008 analysis of test scores across 41 countries published in Science concluded that "data shows a higher variance in boys' than girls' results on mathematics and reading tests in most OECD countries", the results implying that "gender differences in the variance of test scores are an international phenomenon". However, it also found that several countries failed to exhibit a gender difference in variance.
A follow up study of C.S loat et al. 2004 replicated some of their previous findings.
2-4 year olds:
SDQ Prosociality MZF (lower) DZF (higher)
SDQ Hyperactivity MZF (lower)
SDQ problem behaviour MZF (lower)
8 year olds:
CAST composite DZF (higher)
CAST social MZF (lower)
CAST communication DZF (higher)
CAST non-social DZF (higher)
No statistically significant results were found for the cognitive or academic measures. 7 year olds: SDQ conduct, SDQ peer problems, SDQ anxiety. 8 year olds: conners ADHD, RPQ composite.
The study was done on about 1000 twins for each sex and zygosity like the previous study, but the sample was 7–8 years old this time and only different behavioral measures were taken by the 8 year olds unlike the 7 year olds who took cognitive tests too.
2008 review A 2008 study reviewed the history of the hypothesis that general intelligence is more biologically variable in males than in females and presented data which the authors claim "in many ways are the most complete that have ever been compiled [and which] substantially support the hypothesis".
2009 non-cognitive traits study A 2009 study in developmental psychology examined non-cognitive traits including blood parameters and birth weight as well as certain cognitive traits, and concluded that "greater intrasex phenotype variability in males than in females is a fundamental aspect of the gender differences in humans".
2010 metanalysis of math scores Recent studies indicate that greater male variability in mathematics persists in the U.S., although the ratio of boys to girls at the top end of the distribution is reversed in Asian Americans. A 2010 meta-analysis of 242 studies found that males have an 8% greater variance in mathematical abilities than females, which the authors indicate is not meaningfully different from an equal variance. Additionally, they find several datasets indicate no or a reversed variance ratio.
2010 twin study
A 2010 study with a similar methodology to C.S loat et al. 2004 with 272 twin pairs taking an IQ test and 324 twin pairs were assessed using measure of child behavioural problems and found no evidence of an X-linked contribution to greater variance in males and no statistically significant interaction of sex and chorion type.
2012 study on variance in math performance