Sandbox

Nature vs Nurture and Sexuality

For statisticians, Nature vs. Nurture is the gift that keeps on giving. Back in the 19th century it was craniometry. The 20th century focused on intelligence. Now that society is more liberated and less prudish, the spotlight has moved from criminality and ethnicity to sexual preference. One side in the debate claims that the individual’s genetic makeup (nature) determines whether or not an individual is a homosexual while the other side (nurture) alleges that the individual’s homosexuality is due to the undue influence of Hollywood, television, liberalism and, of course, Hillary Clinton.

From here we find

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EXAMPLE A: Hair Whorl (Men) Gay men are more likely than straight men to have a counterclockwise whorl.

This article also contains

http://nymag.com/news/features/gaydar070625_2_560.jpg

EXAMPLE B: Thumbprint Density (Male) Gay men and straight women have an increased density of fingerprint ridges on the thumb and pinkie of the left hand.

as well as

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EXAMPLE C: Digit Proportions (Female)

The index fingers of most straight men are shorter than their ring fingers, and for most women they are the same length or longer. Gay men and lesbians tend to have reversed ratios. and,

http://nymag.com/news/features/gaydar070625_4_560.jpg

EXAMPLE D: Hand Dexterity (Male) Gay men and lesbians have a 50 percent greater chance of being left-handed or ambidextrous than their straight counterparts.

Discussion

1. According to this article, Professor Richard Lippa, a psychologist from California State University at Fullerton, attended “the Long Beach Pride Festival” and gathered “survey data from more than 50 short-haired men and photographed their pates (women were excluded because their hairstyles, even at the pride festival, were too long for simple determination; crewcuts are the ideal Rorschach, he explains). About 23 percent had counterclockwise hair whorls. In the general population, that figure is 8 percent.” See if your favorite librarian can find anything in the literature that indicates that a counterclockwise whorl is found in 8 percent of men.

2. Assume that the 23 percent of all homosexual males have a counterclockwise whirl and 8 percent of the male population has a counterclockwise whirl. Using Bayes Theorem, if you meet a male with a counterclockwise whirl, what is the probability that he is a homosexual. Assume that the homosexual males comprise about 3 percent of the population.

3. Left-handedness, unlike hair whirls, is a fascinating subject in itself and a Google search will turn up many intriguing concepts. Roughly speaking, somewhere around 10 percent of the population is lefthanded. The article indicates that homosexuals “have a 50 percent greater chance of being left-handed” than their straight counterparts. Using Bayes Theorem, if you meet a lefthanded male, what is the probability that he is a homosexual?

4. John T. Manning's book, Digit Ratio: A Pointer to Fertility, Behavior, and Health [Rutgers University Press, 2002] contains over 170 pages devoted to the ratio of the length of the index finger to the length of the ring finger [2D:4D ratio]. He and others believe that the 2D:4D ratio is able to explain such disparate entities as sex and population difference, assertiveness, status, aggression, attractiveness, the wearing of rings, reproductive success, hand preference, verbal fluency, autism, depression, birth weight, breast cancer, sex dependent diseases, mate choice, sporting ability, running speed, spatial perception homosexuality and more. From Professor S.M. Breedlove of Michigan State University: “Animal models have indicated that androgenic steroids acting before birth might influence the sexual orientation of adult humans. Here we examine the androgen-sensitive pattern of finger lengths, and find evidence that homosexual women are exposed to more prenatal androgen than heterosexual women are; also, men with more than one older brother, who are more likely than first-born males to be homosexual in adulthood, are exposed to more prenatal androgen than eldest sons. Prenatal androgens may therefore influence adult human sexual orientation in both sexes, and a mother's body appears to 'remember' previously carried sons, altering the fetal development of subsequent sons and increasing the likelihood of homosexuality in adulthood.” The following data were obtained at gay pride celebrations in the San Francisco Bay Area here and here.

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Figure 1 Finger-length patterns vary with gender, sexual orientation and birth order.

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Fig 2. Finger length ratios in self-identified femme and butch lesbians. Means and standard errors of the means are depicted. A smaller 2D:4D is thought to reflect greater exposure to androgen during the perinatal period. Because the sex difference in 2D:4D is greater on the right hand than on the left (see text), the right hand may provide a more sensitive measure of early androgen than does the left.

Data acquisition can be costly and time consuming. The trick is to find data which is inexpensive to attain and appealing to the general public. Discuss how Lappa, Manning and Breedlove have found an area guaranteed to be successful.

5. In the works of Manning and Breedlove a small p-values is used to claim a result is striking. Given the large sample sizes, why is a small p-value not necessarily impressive? Given the multiple comparisons, why is a small p-value not necessarily impressive?

6. Why are social conservatives unhappy with the implication that homosexuality is innately determined?

Submitted by Paul Alper