Elena Scotti/FUSION

As Kashmir Hill so elegantly puts it, "one of these dots is not like the others":

The chart comes from Mona Chalabi's fantastic post on "the gender orgasm gap". While only 64% of women say that they climaxed during their last sexual encounter, more than 90% of women who received anal sex say that they reached orgasm at some point during the entire sexual encounter.

So, what's going on here? Does anal sex make you come more than other kinds?

Not exactly. Taking a closer look at the data, there seem to be three factors in play.

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First, the sample size of the anal sex portion of the study is tiny: just 31 women. As Chalabi says, the result is that "these numbers are less reliable than the others cited in the chart". Still, they're the best we have to go on. What's more, the pattern is mirrored in men: 100% of the 25 guys who received anal sex also achieved orgasm. (Giving anal sex, on the other hand, was the least likely sex act to result in orgasm for men.)

Related: Why women actually enjoy anal sex

Second, the more sexual acts a woman performs in any given sexual encounter, the more likely she is to reach orgasm. It's fair to assume that most people having anal sex aren't only having anal sex. If anal sex can be used as a proxy for "people who enjoy sex and like to mix up their sex acts a lot", then that can explain why it is so highly correlated with female orgasm. If women who have anal sex are more open to their sexuality and more aware of what gets them to climax, then they will be more likely to get there.

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Finally, to state the obvious, there are no sex organs in your anus: you're very unlikely to come from receiving anal sex alone. But that's not what the data says — it's simply saying that when a woman receives anal sex, she's very likely to have an orgasm within the same sexual encounter.

The most likely explanation, I think, is that anal sex heightens the impetus towards reciprocity, a key concept in all social science, including economics. If someone does something selfless for you, then you feel obliged to do something for them in return. This data is telling us that when a man has anal sex with a woman, the woman is in an economic sense doing a favor for the man. More than any other sex act, anal sex causes a man to be as determined as possible to make sure that the woman has an orgasm.

That outlying purple dot is what you get, in sex, when a man has a strong incentive to return a big favor.