September 20, 2017

Takes two to tango

Right from the start of StatsChat we’ve looked at stories about how men or women have more sexual partners. There’s another one in the Herald as a Stat of the Week nomination.

To start off, there’s the basic adding-up constraint: among exclusively heterosexual people, or restricted to opposite-sex partners, the two averages are necessarily identical over the whole population.

This survey (the original version of the story is here) doesn’t say that it just asked about opposite-sex partners, so the difference could be true.  On average, gay men have more sexual partners and lesbians have fewer sexual partners, so you’d expect a slightly higher average for all men than for all women.  Using binary classifications for trans and non-binary people will also stop the numbers matching exactly.

But there are bigger problems. First, 30% of women and 40% of men admit this is something they lie about. And while the rest claim they’ve never lied about it, well, they would, wouldn’t they?

And the survey doesn’t look all that representative.  The “Methodology” heading is almost entirely unhelpful — it’s supposed to say how you found the people, not just

We surveyed 2,180 respondents on questions relating to sexual history. 1,263 respondents identified as male with 917 respondents identifying as female. Of these respondents, 1,058 were from the United States and another 1,122 were located within Europe. Countries represented by fewer than 10 respondents and states represented by fewer than five respondents were omitted from results.

However, the sample is clearly not representative by gender or location, and the fact that they dropped some states and countries afterwards suggests they weren’t doing anything to get a representative sample.

The Herald has a bogus clicky poll on the subject. Here’s what it looks like on my desktop


On my phone it gets a couple more options visible, but not all of them. It’s probably less reliable than the survey in the story, but not by a whole lot.

This sort of story can be useful in making people more willing to talk about their sexual histories, but the actual numbers don’t mean a lot.


Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient

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  • avatar
    Peter Davis

    In the phone survey I did some time ago males reported twice the number of lifetime partners as females. But when asked to estimate over the last year, there was much less of a difference. So I take this as a combination of recall and social desirability biases. Both genders are struggling to remember, but it is ok for guys to err on the high side but less so for women.

    3 months ago

  • avatar

    You write, “among exclusively heterosexual people, or restricted to opposite-sex partners, the two averages are necessarily identical over the whole population.”

    I have two concerns, which might not be huge but should not be zero either. First is that there’s some subset of the population that is heterosexual now but was not in the past, or vice-versa, and I’d think this could work differently for men and women. Second, there could be time trends interacting with age and cohort. People die, so the average for currently-alive men would not have to be the average for currently-alive women.

    3 months ago

    • avatar
      Thomas Lumley

      The trends thing is a big problem if you try to narrow down the age range for sampling. If you really had population sampling I think there’d be a plateau at older ages and the trends would matter less.

      Still, yes, an issue. And in this case — unlike some previous examples — the news story wasn’t trying to say men were more promiscuous than women or vice versa.

      3 months ago

    • avatar
      megan pledger

      I think there is another issue with “the two averages are necessarily identical over the whole population.”

      What is the “whole population” in this case? With international travel, it would have to be the world.

      3 months ago