The heterosexual men, however, had a more difficult time detecting the same orientation when it was located where an invisible picture of a nude man had been; this was not the case for heterosexual women when viewing their own sex naked. And the homosexual men’s response was similar to that of the heterosexual women, as were the bisexual women’s and heterosexual men’s.

This focus benefit did not carry over, however, when the participants were allowed to consciously see the naked photos, the researchers report in the paper published online in Proceedings of the National Academy of Sciences USA. It may have been that the erotic images were on display too long, they speculate; previous studies have shown that it is difficult to maintain attention in one spot. Or it could be that social or cultural norms take over. "Maybe you don't want to look at the nude pictures," he suggests. Regardless, it appears that our minds are exquisitely tuned to detect sexual opportunity--especially when it is invisible.