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30 March 2000

Nature 404, 455 - 456 (2000) © Macmillan Publishers Ltd.

Finger-length ratios and sexual orientation


Department of Psychology and Graduate Groups Neuroscience, Endocrinology, 3210 Tolman Hall, MC 1650, University of California , Berkeley, California
94720-1650, USA

e-mail: breedsm@socrates.berkeley.edu

Measuring people's finger patterns may reveal some surprising information.

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 lengths1, 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.

In women, the index finger (2D, second digit) is almost the same length as the fourth digit (4D), although it may be slightly longer or shorter; in men, the index finger is more often shorter than the fourth. The greater 2D:4D ratio in females is  established in two-year-olds1. Because all non-gonadal somatic sex differences in humans appearto be the result of fetal androgens that masculinize males3, the sex difference in the 2D:4D ratio probably reflects the prenatal influence of androgenon males4.

In an anonymous survey, 720 adults who were attending public street fairs in the San Francisco area were asked their gender, age, sexual orientation, handedness, and the number and gender of children their mother had carried before them. As expected, men have significantly longer fingers than women (P < 0.001), and we confirmed reports that the 2D:4D ratio is greater in women than it is in men.

This sex difference in 2D:4D is greater on the right hand than on the left (Fig. 1a), indicating that the right-hand 2D:4D is more sensitive to fetal androgens than the left-hand ratio. The right-hand 2D:4D ratio of homosexual women was significantly more masculine (that is, smaller) than that of heterosexual women, and did not differ significantly from that of heterosexual men. Thus finger ratios, like otoacoustic emissions5, suggest that at least some homosexual women were exposed to greater levels of fetal androgen than heterosexual women.

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

2D:4D ratio of homosexual men was not significantly different from that of heterosexual men for either hand (P > 0.09). However, segregating male subjects based on birth order provided support for the role of fetal androgens in male sexual orientation. The more older brothers a boy has, the more likely he is to develop a homosexual orientation2. Confirming these reports, we also found that only homosexual men had a greater than expected proportion of brothers (P< 0.01) among their older siblings (229 brothers:163 sisters) compared with the general population (106 males:100females6).

We found that the male 2D:4D ratio, which is unlikely to be influenced by social factors, also varies with the number of older brothers. The ratio was significantly more masculine in men with two or more older brothers than in men with no older brothers (Fig. 1b). There is also a significant correlation (r = -0.104; P < 0.05) between the number of older brothers and the right-hand 2D:4D ratio in men. If male subjects are divided by sexual orientation, the same pattern of later-born men displaying a more masculine 2D:4D is seen. Having older sisters has no apparent influence on male sexual orientation2, or on the 2D:4D ratio in men. No effect of older brothers or sisters on 2D:4D in women was observed, consonant with reports that older siblings exert no effect on female sexual orientation7.

Our results suggest that events before birth (or even before conception in the case of older brothers) influence human sexual orientation. The masculinized right-hand 2D:4D ratio in homosexual women may reflect fetal androgen levels that are slightly higher than in heterosexual women. Homosexual men without older brothers have 2D:4D ratios indistinguishable from heterosexual eldest sons, indicating that factors other than fetal androgen (such as genetic influences8, 9) also contribute to sexual orientation. Finger measures indicate that men with more elder brothers, including those men who develop a homosexual orientation, might be exposed to greater than normal levels of prenatal androgen.

Although hyper-androgenization of homosexual men might not fit some cultural expectations10, homosexual men display several hyper-masculine characteristics, including a greater mean number of sexual partners in a lifetime than heterosexual men, who in turn report more sexual partners than do women of either orientation. Furthermore, reports that adult homosexual men have more circulating androgens (ref. 11, but see ref. 12), larger genitalia13 and more 'masculine' auditory evoked potentials than heterosexual men14, are consistent with at least some homosexual men being hyper-androgenized.

Although it is possible that the maternal influence on finger growth of subsequent sons occurs after birth, a prenatal influence seems more likely because of the extensive physiological pairing of mother and fetus. The locus of the maternal 'memory' for previous sons, and the mechanisms by which fetal development of subsequent sons is altered, remain unknown.

Manning, J. T., Scutt, D., Wilson, J. & Lewis-Jones, D. I. Hum. Reprod. 13, 3000-3004 (1998). Links
Blanchard, R. Annu. Rev. Sex Res. 8, 27-67 (1997). Links
Breedlove, S. M., Cooke, B. M. & Jordan, C. L. Brain Behav. Evol. 54, 8-14 (1999). Links
Manning, J. T., Trivers, R .L., Singh, D. & Thornhill, R. Nature 399, 214-215 (1999).
McFadden, D. & Pasanen, E. Proc. Natl Acad. Sci. USA 95, 2709-2713 (1998). Links
James, W. H. Hum. Biol. 59, 721-752 (1987). Links
Bogaert, A. F. Behav. Neurosci. 111, 1395-1397 (1997). Links
Bailey, J. M. & Pillard, R. C. Arch. Gen. Psychiatry 48, 1089-1096 (1991). Links
Hamer, D. D., Hu, S., Magnuson, V. L., Hu, N. & Pattatucci, A. M. L. Science 261, 321-327 (1993). Links
Gorman, M. R. Persp. Biol. Med. 38, 61-81 (1994).
Brodie, H. K. H. et al. Am. J. Psychiatry 131, 82-83 (1974). Links
Mayer-Bahlburg, H. F. L. Progr. Brain Res. 61, 375-398 (1984).
Bogaert, A. F. & Hershberger, S. Arch. Sexual Behav. 28, 213-221 (1999).
McFadden, D. & Champlin, C. A. J. Ass. Res. Otolaryngol. (in the press).

NewsWeek Press Release

Let Your Fingers Do the Talking

By B. J. Sigesmund
Newsweek, March 31, 2000

For Marc Breedlove, all it took was a Xerox
machine and a dream.

The University of California at Berkeley professor, who studies the biology of sexual orientation, took his copier and a few assistants to three street fairs in San Francisco last fall. His quest: to see if hormones in the womb might play a role in sexual behavior. At each event, he offered passersby a $1 lottery ticket in exchange for information regarding their sexual orientation and their birth order--and a copy of their palmprint. The psychologist collected data from 720 people.

The results, published this week in the journal Nature, stir the debate over whether homosexuality is learned, influenced or fixed by prenatal factors. Scientists have long believed that finger lengths may indicate the levels to which a fetus was exposed to male sex hormones, such as androgen, while in the womb. Now Breedlove's research indicates that both women's and men's finger lengths might signal sexual orientation, suggesting that hormonal exposure is one determining factor in sexuality. "We expected any such effects to be subtle because so many factors play a role in human behaviors," he says. "So we were excited to find [some] evidence."

Breedlove's team found that the women interviewed who identified themselves as lesbian tended on average to have "finger-ratios" that were more like men's. A person's finger-ratio is the index finger's length divided by the ring finger's length. In men, the average ratio is 0.95. For women, 0.97. Lesbians who participated, the Berkeley team found, scored 0.96.

The study of the finger-ratios of straight men and gay men who were interviewed proved more complicated. Breedloves researchers found no difference between the straight and gay men's finger-ratios until they separated the men by birth order. Then they found that gay males who had two or more older brothers had slightly lower ratios than those with no older brothers. And lower ratios in males are a possible indication of homosexuality, Breedlove believes.

Breedlove says his study corresponds to the work of an associate in his field, Ray Blanchard, who's shown that boys who have greater numbers of older brothers have a higher likelihood of being gay. Blanchard says he thinks Breedlove's study furthers the theory that hormones in the womb have an effect on the eventual sexual orientation of the fetus. "You can't learn your finger length, and your finger length is not a lifestyle choice," Blanchard says.

What's next? In order to further validate the research, another researcher must replicate Breedlove's exact findings in a second survey of both straight and gay people. Blanchard believes it won't be long. "You don't need an atom smasher," he says, "only a photocopier." Some lottery tickets wouldn't hurt either.

From the New Scientist Web Page

Do a person's fingers reveal their sexual

YOUR early life as a fetus may have influenced your sexuality as an adult--and your fingers tell part of the story. Researchers in California have used relative finger lengths to show that sexual orientation is partly determined by events in the womb.

In animals, prenatal exposure to the male sex hormone testosterone seems to influence sexual orientation. But it is not easy to measure fetal hormone levels in humans. One indirect way is to look at the size of a person's fingers. In women, the index finger, called the second digit or 2D, is about the same length as the ring finger, 4D. In men, the ring finger is often considerably longer, leading to a lower 2D:4D ratio. This sex difference is clear from infancy, and researchers attribute it to masculinising hormones during fetal development.

Marc Breedlove and his colleagues at the University of California at Berkeley wanted to know if gay people had different finger length ratios from straight people, so they surveyed 720 adults during street fairs in San Francisco. The researchers collected information about gender, age, sexual orientation, handedness and older siblings. They also carefully measured the lengths of the volunteers' fingers.

They found something striking in gay women: their index to ring finger ratios resembled those of heterosexual men. This suggests that at least some lesbian women were exposed to higher than average levels of male hormones before birth.

What they found in men is less clear-cut. The 2D:4D ratio in gay men was not significantly different from that in straight men. But a series of studies in the 1990s had shown that the more older brothers a boy had, the more likely he was to be gay, so the researchers sorted the volunteers according to numbers of older brothers. They found that all the men who had two or more older brothers had significantly smaller 2D:4D ratios. "It was a big surprise to me that the finger measures would follow the epidemiology so closely," says Breedlove.

The findings suggest that homosexuality is partly due to higher levels of prenatal testosterone in men as well as women, he says. But they also show that fetal hormones alone don't determine sexuality. First-born males have indistinguishable 2D:4D ratios, whether they are gay or straight, yet some first-born males are gay. So other factors clearly come into play, says Breedlove.

To some researchers, the idea that gay men are "hypermasculinised" seems counterintuitive. John Manning of the University of Liverpool has found the opposite in gay men: that their finger ratios veer more towards the feminine. He suspects that both very low and very high levels of testosterone in the womb could produce homosexuality. "There may be more than one phenotype of male gays," he says.

Manning also wonders if Breedlove's data may have been slightly muddied because the research did not take account of ethnicity. He has found big population variations in 2D:4D ratios. "The geographical differences swamp the sex differences," says Manning. "There's more difference between a Pole and a Finn than between a man and a woman."

Interview with Marc Breedlove


We did not find a difference between gay and straight men in ring-to-index-finger ratios, but we did find a more masculine ratio in men who had more than one older brother when compared to men who had no older brothers, which suggests that the more older brothers you have, the greater your testosterone exposure in the womb. This is especially interesting because there are a lot of data sets that look at men’s sexual orientation and number of older brothers, and all of them show that gay men tend to have more older brothers than the rest of the population. For the rest of the population, the ratio is 106 brothers to 100 sisters, whether older or younger. In our survey, gay men had 140 older brothers for every 100 older sisters, but for younger brothers and sisters, for gays the figure is also 106 to 100, as with the rest of the population.

Blanchard estimates that about fifteen percent of the men who are gay in North America are gay because they have older brothers. If their mother had had fewer boys before them, they would be straight today. Fifteen percent is a significant number. And think of it politically. Why on earth should people have hundreds of fewer legal rights and be unable to marry the person they fall in love with just because their mom had sons before them?

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(2010-03-29 17:55)



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