The male orgasm plays an obvious and important part in sexual reproduction, given that it typically co-occurs with ejaculation. Orgasm is therefore pretty critical to how men “spread their seed.” But what about the female orgasm? Does it also play a role in reproduction, or does it perhaps exist for some other reason?

Scientists have been debating this for decades, and they’ve come up with at least three different theories.

Some have argued that orgasming during sex increases women’s odds of getting pregnant. How so? The thought here is that orgasm causes release of the hormone oxytocin, which leads to contractions of the uterus. These contractions supposedly “suck” more sperm into a woman’s upper reproductive tract, thereby making it easier for fertilization of an egg to occur.

As support for this idea, consider a 1993 study in which women were asked to collect semen that seeped out of their vaginas after sexual intercourse. (Oh, the things people will do in the name of science…)

Researchers then compared the amount of sperm present in the semen samples obtained from women who did and did not have orgasms.

What they found was that those women who had orgasms just before or after their partners “ejected” less sperm after sex, compared to women who didn’t orgasm at all.

Although fascinating, this study provides only indirect support for the theory, and it has a lot of limitations. For example, because we don’t know how much sperm was actually released in each ejaculation, we can’t draw firm conclusions about how much was retained, let alone whether uterine suction is what accounted for any differences.

The truth of the matter is that we don’t yet have any conclusive evidence for the upsuck theory.

Besides, if the female orgasm really did facilitate reproduction in this way, than wouldn’t we expect women to have more consistent orgasms?

A different theory is that orgasms help women to identify high-quality male partners. From this perspective, the fact that women’s orgasms are less consistent than men’s is because they aren’t “triggered” as easily by every guy.

The basic idea of this theory is that women are most likely to have orgasms with guys who have higher “mate value.” By this, I mean guys who have good genes and the necessary resources to provide for any children they might father.

In this sense, orgasms are essentially a reward for finding a high-quality mate and a signal that he’s worth sticking with.

As evidence for this theory, studies have found that women are more likely to report having orgasms with guys who are “good catches” (i.e., physically attractive guys with money)—and the more orgasms they report having, the more satisfied they tend to be.

I’m not sure I buy the mate-choice explanation entirely, though, especially in light of other research finding that women’s likelihood of orgasm has more to do with a certain anatomical trait—the distance between the clitoris and the vagina—than anything.

To me, this suggests that women’s orgasms may say more about their genital anatomy than the men they’re with.

Yet another theory is that the female orgasm has no adaptive value at all. Instead, it’s just an evolutionary byproduct—a “fantastic bonus,” if you will, for women.

The idea here, advanced by Dr. Elisabeth Lloyd in her 2005 book The Case of the Female Orgasm, is this: in the early stages of fetal development, we all have the same tissue structures, and these structures have the potential to develop toward either the male or female form depending upon the hormones that we’re exposed to.

These tissues are organized such that when development proceeds in the male direction, the capacity for orgasm is guaranteed to emerge because it’s essential for future reproduction.

Because biology favors development of the male orgasm so strongly, Lloyd argues that the female orgasm is nothing more than a byproduct of this.

A parallel can be drawn here to the male nipple.

Biology heavily favors development of female nipples because they’re essential for nourishing children through breastfeeding. According to Lloyd, the male nipple can therefore be explained as an evolutionary byproduct, too.

Of course, men’s nipples might also rightfully be described as a “fantastic bonus,” given that most men find nipple stimulation to be sexually arousing.

It’s hard to say without more research. And even with a lot of future study, we may never know with certainty. In my mind, though, the current evidence for the adaptive explanations leaves something to be desired. So, at least for the time being, I’ve got my money on the fantastic bonus.

Justin Lehmiller, PhD is a sex educator and researcher at Ball State University and author of the blog Sex and Psychology. Follow him on Twitter @JustinLehmiller.