Thursday, August 16, 2007

Lies, Damn LIes, and the Number of Sexual Partners

A few days ago, Gina Kolata reported in the New York Times on the paradox of studies on sexual behavior consistently reporting (heterosexual) men having more sexual partners than women, with a recent US study reporting men having a median number of 7 partners and women a median number of 4. Contrary to what's stated in the paper, this is not mathematically impossible (key word: median). It is however quite implausible, requiring a relatively small number of women to account for a large fraction of all men's partners.

An answer to this paradox can be found in Truth and consequences: using the bogus pipeline to examine sex differences in self-reported sexuality, by Michele Alexander and Terry Fisher, Jorunal of Sex Research 40(1), February 2003.

In their study, a sample of men and women are each divided into three groups and asked to fill a survey on sexual behavior. People in one group filled the survey alone in a room with an open door, a researcher sitting outside, and after being told the study was not anonymous; people in a second group filled the survey in a room with a closed door and an explicit assurance of anonymity; people in a third group filled the survey attached to what they believe to be a working ``lie detector.''

In the first group, women reported on average 2.6 partners, men 3.7. In the second group, it was women 3.4 and men 4.2. In the third group, it was women 4.4 and men 4.0.

(The study looks at several other quantities, and some of them have even wider variance in the three settings.)

So, not surprisingly given the sexual double standards in our culture, men and women lie about their sexual behavior (men overstate, women understate), and do less so in an anonymous setting or when the lie is likely to be discovered.

Here is the reporting of the first group put to music:

[Update 8/18/07: so many people must have emailed her about the median versus average issue in the article that Gina Kolata wrote a clarification. Strangely, she does not explain, for the rest of the readers, what the difference is and why it is possible, if unlikely, to have very different medians for men and women. The claim in the clarification, by the way, is still wrong: those 9.4% of women with 15 or more partners could be accounting for all the missing sex.]


  1. Anonymous Anonymous
    8/17/2007 01:23:00 AM

    Luca, if you had watched the fine piece of cinema that is American Pie 2, you would have already known about the "Rule of Three"

  2. Anonymous Anonymous
    8/17/2007 07:23:00 AM

    The only honest answer could be zero.

  3. Blogger Luca
    8/17/2007 03:15:00 PM

    I am amazed that there are people who watch American Pie 2 and read In Theory.

  4. Anonymous Anonymous
    8/17/2007 05:03:00 PM

    men overstate

    Except when they don't. According to your numbers, men reported lower numbers when they didn't believe the setting to be anonymous. How can that possibly be construed as overstating? Please, do tell.

  5. Anonymous Anonymous
    8/17/2007 07:58:00 PM

    I am amazed that there are people who watch American Pie 2 and read In Theory.

    Fair enough. But we all agree that American Pie (the original) is a classic, right?

  6. Blogger Luca
    8/18/2007 11:01:00 PM

    Anonymous #4: you caught me overstating my claim, thus, by the way, proving my point.

  7. Anonymous Anonymous
    8/19/2007 06:06:00 PM

    Except that the point was that men don't overstate.

  8. Blogger ton4eg
    8/20/2007 03:03:00 AM

    Oh, I also recall about american pie 2

  9. Blogger Scott
    8/21/2007 06:50:00 PM

    I also thought immediately of American Pie 2. (Not that I've ever seen it, you understand; I was merely told about it. Is there also a "Rule of Three" for movies?)

    In the "lie-detector" study, is it still the case that a tiny proportion of women bring the average up a lot? That could explain the remaining difference in the medians, and wouldn't be so surprising sociologically either.

    What does surprise me is that Gina Kolata wouldn't have explained the difference between mean and median: her mother was both a noted mathematician and an Aaronson. :-)

  10. Anonymous Anonymous
    8/27/2007 06:20:00 AM

    It is possible that the empirical means
    for men and women differ. These would
    be the means of small sample populations.
    The sampling might exclude a tiny highly
    promiscuous group.


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