People who identify as bisexual have long had to contend with the phrase, "You're either gay, straight, or lying." In 2005, this belief got a big dose of media attention when it formed part of the title of a New York Times report on new research into sexual arousal. Psychologists had used a device called a plethysmograph to measure men's erections, and found that men who said they were bisexual were aroused (at least genitally) by images of men or women, but not both. Benedict Carey of the Times wrote that the study "casts doubt on whether true bisexuality exists, at least in men."

That study has come under fire in recent years (and one of its authors got in trouble for organizing an in-class demonstration of something called a "fucksaw"). But now there's direct evidence against its main conclusion: that bisexual men show no physical signs of attraction to both sexes. That evidence comes in the form of a study of pupil dilation.

Some preliminary research has linked pupil dilation to sexual arousal, and measuring pupils has some big advantages over measuring people's genitals: most notably, people are much more likely to submit to having their eyes examined then they are to having a scientific device stuffed down their pants. People may also be able to control their genital arousal, but it's much more difficult, it turns out, to control one's pupils.

The study authors, psychologists Gerulf Rieger and Ritch C. Savin-Williams, and their team asked 165 men and 160 women to record their sexual orientation — if they were bisexual, they also had the option of listing which sex they were more attracted to, if any. Then the researchers showed them videos of either a man or a woman masturbating (the videos "were selected from a large pool of videos drawn from sites on the Internet"). As it turned out, bisexual men showed pupil dilation in response to both videos of men and women — and, crucially, they showed significantly more dilation in response to the sex they were less attracted to than straight or gay men did to the sex they weren't attracted to at all. That is, bisexual men did display a bisexual pattern of physical arousal — at least as measured by their eyes.

There's more to sexuality than physical arousal — for instance, the study authors note that some men might "identify as bisexual not because they show bisexual arousal but because they have distinct personalities that open them to a variety of sexual experiences, including sexual experiences with the less preferred sex." And no laboratory measure of arousal is perfect. Still, there's now direct evidence against the idea that bisexual men show no physical evidence of their orientation. And this may help put an end to the notion of "straight, gay, or lying."