US: There are odours that drive a person's sexual response, and scientists have found that homosexual men differ from heterosexual men in the way they respond to such smells.
Their brain activity more closely resembles the responses observed in women, new research has shown.
"This is another piece of evidence that the brains of gay and straight people are organised differently," said Simon LeVay, a biologist who almost 15 years ago identified a structural difference in the brains of homosexual and heterosexual men.
"It is a fascinating finding, but still doesn't explain the origins of sexual orientation," said Dr LeVay.
The new work, by Ivanka Savic and her colleagues at the Karolinska Institute in Stockholm, grew out of an earlier finding that brain scans of men and women differed dramatically from each other in response to chemical smells that mimic male and female hormones.
In their latest study, in the Proceedings of the National Academy of Sciences, the researchers put gay men into a brain-scanning machine and saw that the region of the anterior hypothalamus became activated when the men were given male sex hormones to smell.
By contrast, heterosexual men showed more brain activity in this region when smelling odours associated with female hormones.
The smells used in the Swedish study were made by reconstructing chemicals in male perspiration and female urine that mimic derivatives of testosterone and oestrogen.
In the current study, the brains of the homosexual men showed similar responses to that of heterosexual women when sniffing testosterone.
The brains were no different from those of heterosexual men and women when responding to a nonsexual smell, lavender.
The homosexual men reacted to oestrogen in the same way as they responded to lavender, Dr Savic said.
She said her finding did not suggest that they found a difference in the brain itself, "just that the response is different".
She added that she did not look at the structure of the hypothalamus in this study.
They are now repeating these studies in homosexual women and planning to add transsexuals to the mix as well.
Dr Sandra Witelson, a neuroscientist at the Michael G DeGroot School of Medicine at McMaster University in Hamilton, Ontario, said the research showed "that there is a neurobiological difference between gay and straight men". - (LA Times-Washington Post Service)