Finger length may be an indication of sexual orientation, a controversial study has shown.

Scientists from California found that lesbian women have a greater difference in length between their ring finger and index finger than straight women do.

The same pattern was also found for homosexual men - but only when the researchers looked at those males that had several older brothers.

The scientists from the University of California at Berkeley were testing a theory that higher levels of androgen - male sex hormones - in the womb influence both finger length and sexual orientation.

They did this by looking at the hands of 720 men and women on the streets of San Francisco. The volunteers had their fingers measured and were asked questions about their sexual orientation and the number of older brothers and sisters in the family.

In women, the ring finger and index finger tend to be about the same length. In men, however, the index finger is usually the shorter of the two digits.

'Masculine' pattern

What the study showed was that lesbian women also tended to have the more "masculine" arrangement - that is, they had shorter index fingers.

But the ratio of finger sizes in men was more complicated. Comparisons between all men showed no differences. Only gay men with several older brothers had an unusually "masculine" finger ratio - in other words, they had significantly shorter index fingers.

Having a large number of older brothers had previously been established as a factor predisposing men to homosexuality, and like finger length reflects prenatal androgen exposure.

Homosexual men without older brothers had finger length ratios indistinguishable from heterosexual men, indicating that factors other than hormones - such as genetic influences - also contribute to sexual orientation.

"The results in men are more complicated but also more interesting," Dr Marc Breedlove, one of the researchers, told the BBC. This is because they suggest younger brothers are being exposed to higher levels of androgen in the womb than their elders.

Womb 'memory'

"We think it is inescapable that the mother's body is remembering how many sons she has carried before, and somehow she is then increasing the amount of androgen that each subsequent son sees before birth. So the fascinating questions are: where is the memory being stored in the mother's body and what is she doing to change amount of androgen that each subsequent son sees."

The Berkeley study has been published in Nature. Scientists in the UK have given it a cautious reception. They say far more work is required to back up some of the ideas contained in the research.

"I think this is a possibility," said Dr Richard Sharpe, of the human reproductive science unit at Edinburgh University. "But no-one has actually measured the levels of androgens in foetuses or the womb - you can imagine it is difficult to do - so this is all speculation. It may be informed speculation, but I think when we get on to a subject as touchy as sexual orientation we need to clearly define what is speculation and what is fact."

Dr Sharpe said animal studies had shown how female sex hormones could affect sexual differentiation in the brain and regulate sexual behaviour, but this had still to be proved in humans. It could well be different, he said.