The length of a child's fingers can point to whether he or she is likely to be gay in later life, according to a study linking finger patterns to sexual orientation.
Scientists believe the findings reveal a link between levels of exposure to sex hormones during foetal development and established differences in finger lengths between men and women.
In women, the index finger - or forefinger - is typically almost the same length as the ring finger, or fourth digit. In men, the index finger is more often shorter than the ring finger. The effects tend to be greater on the right hand.
This difference between the sexes is established at the age of two. Because all sex differences in pre-pubescent children are the result of exposure to male sex hormones, or androgens, in the womb, scientists believe finger differences are also due to differential exposures to foetal androgens.
The new study indicates that the relative lengths or ratios of the index finger and ring finger can also predict whether a woman is likely to be a lesbian and whether a man is likely to be homosexual, provided the number of his elder brothers is taken into account.
"The results suggest that events before birth influence our sexual orientation in adulthood," said Marc Breedlove, a psychologist at the University of California, Berkeley, who led the study, published in Nature.
"The effects are subtle, so you cannot accurately classify individuals' orientations, or even their sex, based on finger ratios. But the law of big numbers tells us that even such subtle effects, when applied to large populations, will make a difference," he said.
Interviews with 720 people who had their fingers measured showed that women who said they were lesbians had a more masculine ratio of finger lengths, with a shorter index finger relative to ring finger. This suggests that gay women were on average exposed to greater levels of androgens in the womb than heterosexual men.
"The results were more complicated, but more interesting in men," Professor Breedlove said. "Gay men as a group did not differ from straight men. But if we classify men based on a factor known to influence orientation, birth order, we see a difference."
Earlier work by Ray Blanchard, of Toronto University, found that the greater the number of older brothers a boy has, the greater his chances of being gay in adulthood. Even with six or seven older brothers, the probabilities are still low enough for most of these boys to be heterosexual. But the effect, although subtle, has been repeatedly shown.
Professor Breedlove said: "We found that men with older brothers also had more masculine finger ratios than did eldest sons.
"So we think that the more older brothers a boy has, the more foetal androgens the boy will be exposed to.
"While the majority of such later-borns grow up to be heterosexual, they are more likely than first-borns to be gay and we think the increased levels of foetal androgen are responsible for this increased probability. I think there are some men out there who are gay because they were later-born. That is, if they had received the same genes, same family, same neighbourhood etcetera, but been their mother's first son, they would be straight today."