1. The History of the Variability Hypothesis

2. Leta Hollingworth's studies

3. Modern studies

4. See also

5. References

The variability hypothesis, also known as the greater male variability hypothesis, states that males display greater variability in traits than females do. It has often been discussed in relation to cognitive ability, where it has been observed that human males are more likely than females to have either very high or very low intelligence. The sex-difference in the variability of intelligence has been discussed since at least Charles Darwin.^[1] Sex-differences in variability are present in many abilities and traits - including physical, psychological and genetic ones. It is not only found in humans but in other sexually dimorphic species as well.

The History of the Variability Hypothesis

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 The Descent of Man and Selection in Relation to Sex, Darwin noted that in many species, including humans, males tended to show greater variation than females in sexually selected traits:

To exemplify this greater male variability in humans, Darwin also 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 man’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 Havelock Ellis. In his 1894 publication Man and Women: A study of human sexual characters, Ellis dedicated an entire chapter to the subject, entitled “The Variational Tendency of Men”^[3]. 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

In the early twentieth century, the variability hypothesis flourished in sociological, psychological, medical and educational literature. Among the proponents of the variability hypothesis at this time were psychologists G. Stanley Hall, Edward Lee Thorndike, and James McKeen Cattell.^[5][6][7] One logical conclusion drawn from the variability hypothesis was that since women were not expected to exhibit above-average intelligence, it was unreasonable to expect eminence from them. This led Thorndike and Hall to suggest the adoption of curricula aimed at preparing women for their future roles as mothers and wives.^[8] The only significant critic of the variability hypothesis was Karl Pearson, a British psychologist who had studied variability in 1897 and found no sex differences.{{citation needed|date=September 2018}} Pearson's research was the only published scientific investigation of the variability hypothesis prior to the work of American psychologist Leta Hollingworth.^[5][6][8]

Leta Hollingworth's studies

Hollingworth's position at the Clearing House for Mental Defectives allowed her the opportunity to refute the variability hypothesis. By examining the case records of 1,000 patients, 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.^{[5][6][8][9][7]}

Hollingworth also attacked the variability hypothesis theoretically, criticizing the underlying logic of the hypothesis.^{[5][6][8][10]} 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.^[5][6][8] Additionally, the argument that great variability automatically meant greater range was criticized by Hollingworth.^[8][11]{{how|date=September 2018}}

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, and it was concluded that if variability "favoured" any sex it was the female sex.^[5][6][8][9] Additionally, along with the anthropologist Robert Lowie, Hollingworth published a review of literature from anatomical, physiological, and cross-cultural studies, in which no objective evidence was found to support the idea of innate female inferiority.^{[5][6][8][11][10]}

Hollingworth's doctoral dissertation also dealt with the psychology of women. Entitled "Functional periodicity: An experimental study of the mental and motor abilities of women during menstruation", it found no evidence of changes in performance associated with phases of the menstrual cycle, refuting a common belief of the time.^{[5][6][8][9][11][7][10][12][13]} Hollingworth's graduate supervisor was E. L. Thorndike, himself a supporter of the variability hypothesis.^[5][6][7]

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.^[14][15] In general, most researchers have failed to find greater variability in either sex in any area, that is consistent across cultures and not confounded by social factors.^{[16][17][18][19]}

Recent studies shows 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 some specific immigrant groups.^[20]

A 2014 review found that males tend to have higher variance on mathematical and verbal abilities but females tend to have higher variance on fear and emotionality; however, the differences in variance are small and the causes remain unknown.^[21] A 2005 meta-analyses found greater female variability on the standard progressive matrices, and no difference in variability on the advanced progressive matrices,^[22] and a 2007 meta-analysis found that males are more variable on most measures of quantitative and visuospatial ability.^[23] Moreover, 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”^[24]

2010 meta-analysis of 242 studies found that males have an 8% greater variance in mathematical abilities than females.^[25] A 2012 study of standardized scores across 86 countries showed that variance was consistent within a given country, but differed between different countries, indicating that variance could be driven by sociocultural factors.^[26] A 2013 study of gender differences in creative thinking in mainland China generally supported the hypothesis that boys have greater variability than girls in creativity test performance.^[27] A 2016 study of twelve databases from the International Association for the Evaluation of Educational Achievement and the Program for International Student Assessment were used to analyze gender differences within an international perspective from 1995 to 2015, and concluded, “The ‘greater male variability hypothesis’ is confirmed.”^[28]

A 2017 literature review by Sean Stevens for the Heterodox Academy found that males do tend to be more variable than females on a variety of measures of intelligence and personality and that gender-egalitarian countries demonstrate the greatest differences.^[29] A 2019 meta-analysis of sex differences in variability across nations in reading, mathematics and science confirmed that variability is greater for males internationally.^[30]

See also

• Sex differences in humans
• Sex and psychology
• Bateman's principle
• Lek paradox

References

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