As a sex researcher, it's helpful for me to know the sexual orientation of the people I study. However, measuring sexual orientation isn't as simple or straightforward as most people might think.

The most common way of measuring sexuality is to simply ask people about it on a survey, but this method poses some problems: One is figuring out which questions to ask. For example, do you focus on people's sexual identities? Do you ask about which genders they're attracted to? Or do you instead ask about the gender of their previous sex partners?

The answers to these questions don't always line up in the way you might expect. For example, some people who identify as heterosexual report same-sex attractions and behaviors. To be clear, this doesn't necessarily mean they're lying. After all, some people are just more sexually fluid than others—a fact that makes the measurement of sexuality inherently complex.

Of course, not everyone is honest on sex surveys, and that serves to complicate matters even further. For instance, some people who have a history of same-sex attraction or behavior will lie or fail to report it. This means that even if you ask the right questions, you're not necessarily going to get valid responses. The challenges posed by sexuality surveys have led many scientists to look for more objective ways of assessing sexual orientation. Some have gravitated to genital arousal measures, in which they hook high-tech devices up to people's genitals, show them a bunch of porn, and record what goes on "down there."

Some sex scientists swear by these measures because "genitals don't lie." However, some research has found that people can fake their genital responses, such as by thinking about unpleasant and gross things while they're watching porn. Findings like this mean that these measures definitely aren't infallible. There's also the matter of who's willing to participate in genital arousal studies: Is it just people with more exhibitionistic tendencies? If so, how generalizable are the results of these studies?

On top of all that, a not insignificant number of people who take part in genital arousal studies are deemed genital "non-responders," meaning they don't register enough arousal in response to any kind of porn to yield usable data. This may be because they didn't find any of the porn appealing, or maybe because they found the whole setup to be a little too anxiety-inducing to get turned on. Either way, it raises more questions about the validity of this method.

That's why some researchers have begun to explore another method: pupil dilation. Scientists have known for more than a half-century that our pupils dilate whenever we see something we find sexually arousing. This is a non-voluntary response that is controlled by the autonomic nervous system—the body system that controls other automatic responses like heart rate, respiration, and digestion. As a result, it's not something that people can consciously control, so it gets around the problems of people lying on surveys and faking genital responses. It also doesn't require people to remove clothing or attach devices to their genitals, which makes it less of a hard sell to research participants.

More From Tonic:

Though scientists have known about these pupil dilation effects for a long time, the first study to use it as a measure of sexual orientation in both men and women didn't appear until 2012. In this study, published in the journal PLoS ONE, participants watched video clips of either a man or woman masturbating, interspersed with nature videos. What they found is exactly what you'd expect: Heterosexual men dilated the most when they saw the female video, gay men dilated the most when viewing the male video, and bisexual guys dilated a lot in response to both kinds of clips.

Likewise, lesbians dilated the most to the female video and bisexual women dilated in response to both videos. However, heterosexual women showed the bisexual pattern, too. That's not surprising, though, considering that earlier genital arousal studies of straight women found the same thing. We don't know for sure why, but some scientists think it's because women evolved to have a more "flexible" sexuality than men.

A 2015 study published in the journal Biological Psychology largely replicated these results. I should also say that this study measured both genital arousal and pupil dilation at the same time and found that what the pupils were doing correlated strongly with what the genitals were doing, especially among men. Flash forward to this year and a new set of studies published in the Journal of Sex Research finds that—among men—pupil dilation doesn't just reveal the gender of the people they're attracted to, but also their age.

In this research, gay, straight, and bisexual men were shown images of both male and female adults and children wearing swimwear on a beach while an eye-tracking device recorded their pupillary responses. At the start of the study, all of these men said they were exclusively attracted to adults and had no attraction to children. In response to the images of adults, participants' pupils followed the same pattern as the previous studies—in other words, their pupils lined up with their sexual orientation.

However, when viewing images of children, no increase in pupil size was observed for men in any sexual orientation group. In fact, if anything, their pupils actually had a tendency to constrict when viewing kids. The results of all of these studies suggest that pupil dilation holds a lot of promise as a technique for measuring attraction based on both gender and age. This could prove useful to scientists who want to measure sexual orientation while avoiding the inherent problems in surveys and genital arousal measures. However, it could also be quite useful to forensic researchers who study pedophilia and other sexual interests that people are strongly motivated to conceal.

Justin Lehmiller, PhD is the director of the social psychology program at Ball State University, a faculty affiliate of The Kinsey Institute, and author of the blog Sex and Psychology. Follow him on Twitter @JustinLehmiller__.