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The variability hypothesis originated in the early nineteenth century with Johann Meckel, a German anatomist, who argued that females are more variable than males. Because he considered males to be the "superior animal" Meckel concluded that variation was a sign of inferiority.   Later that century the character of the variability hypothesis was altered to coincide better with Charles Darwin's emphasis on the importance of variation from the average for the evolutionary process. Following Darwin, the variability hypothesis was instead put forward as man's greater variability compared to women and its application was soon extended to sex differences in mental abilities.    Thus, men were thought to vary greatly in their abilities, while women were assumed to be basically the same in their abilities. The greater number of men at both ends of the intellectual spectrum (as patients in institutions and as great intellectual achievers) was taken as evidence of man's greater innate variability.      
In the early twentieth century the variability hypothesis flourished in sociological, psychological, medical and educational literature. The hypothesis received support not from empirical evidence, but rather from "armchair dogma" about innate female inferiority. Among the proponents of the variability hypothesis at this time were psychologists G. Stanley Hall, Edward Lee Thorndike, and James McKeen Cattell.    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.  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. Pearson's research was the only published scientific investigation of the variability hypothesis prior to the work of American psychologist Leta Hollingworth.   
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.     
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.    Additionally, the argument that great variability automatically meant greater range was criticized by Hollingworth.  
In an attempt 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 neonates. 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.     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.     
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.          Interestingly, Hollingworth's graduate supervisor was E. L. Thorndike, himself a supporter of the variability hypothesis.   
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 Benjamin, L. T. (1975). The pioneering work of Leta Hollingworth in the psychology of women. Nebraska History, 56, 493-505.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 Benjamin, L. T. (1990). Leta Stetter Hollingworth: Psychologist, educator, feminist. Roeper Review, 12, 145-151.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 Shields, S. A. (1975). Ms. Pilgrim’s Progress: The contributions of Leta Stetter Hollingworth to the psychology of women. American Psychologist, 30, 852-857.
- ↑ 4.0 4.1 4.2 4.3 Benjamin, L. T., & Shields, S. A. (1990). Leta Stetter Hollingworth (1886-1939). In A. N. O’Connell & N. F. Russo (Eds.), Women in psychology: A bio-bibliographic sourcebook (pp.173-183). Westport, Connecticut: Greenwood.
- ↑ 5.0 5.1 5.2 5.3 Denmark, F. L., & Fernandez, L. C. (1993). Historical development of the psychology of women. In F. L. Denmark & M. A. Paludi (Eds.), Psychology of women: A handbook of issues and theories (pp. 1-22). Westport, Connecticut: Greenwood.
- ↑ 6.0 6.1 6.2 6.3 6.4 Shields, S. A. (1991). Leta Stetter Hollingworth: "Literature of Opinion" and the study of individual differences. In G. A. Kimble, M. Wertheimer, & C. White (Eds.), Portraits of pioneers in psychology (pp.243-255). Washington, DC: American Psychological Association.
- ↑ 7.0 7.1 7.2 Poffenberger, A. T. (1940). Leta Stetter Hollingworth: 1886-1939. The American Journal of Psychology, 53, 299-301.
- ↑ Gates, A. I. (1940). Leta S. Hollingworth. Science, 91, 9-11.
- ↑ Shields, S. A., & Mallory, M. E. (1987). Leta Stetter Hollingworth speaks on "Columbia’s legacy". Psychology of Women Quarterly, 11, 285-300.