Do men have more partners than women?

Three effects I can see that haven't been mentioned yet:

Prostitutes have a significantly lower life expectancy than non-prostitutes - a mortality rate 40 times as high. Overall mortality rate for, say, a 25 year old woman, is about 1 in 2000. For prostitutes, it's thus about 1 in 50. Compound that for, say, ten years, and you get only 82% of prostitutes surviving past that point.

The lowest-class prostitutes would service the most men and have the highest mortality rate. If they have a 10% mortality rate per year (only five times worse than the average across all prostitutes) over a career of 10 years, then the whole cohort of women older than that point will have about 65% of them dead. After observing the horror of this, we note that death also renders one unable to respond to surveys.

Second effect: prostitutes especially but anyone highly promiscuous in general may have a hard time remembering just how many partners they've had, and may be poor at estimating it, even if they're not trying to minimize it. Especially when they need to discount for repeat customers by a somewhat difficult to estimate margin... it's a genuinely tough problem when you get up into the professional levels, at least without bookkeeping, which will frequently not be kept.

Third effect: prostitutes and ex-prostitutes, being especially disadvantaged, may be harder to reach.

Something I've always wondered about when hearing this 'statistic', but never actually checked - does the gap disappear if you are careful in defining sexual partner?

eg, if the question asked "with how many people have you had consensual penetrative heterosexual intercourse?" rather than "how many sexual partners have you had?"

Sounds about right. I would imagine about half the women I have had sex with would not admit to having had sex with me.

One of taw's old posts is relevant here; his list of possible skewing factors even overlaps with yours quite a bit! He also found a study of students suggesting that the women's reported number of sexual partners changed when they were hooked up to a fake polygraph or thought an experimenter might see their answers. (See also the comments for other explanations.)

Today I came across an article in the Telegraph that states that an average man has 9 sexual partners in his lifetime, whilst a woman has only 4.

Did they include female prostitutes in their samples, when they were calculating said "averages"?

Here's a similar situation: I have read two papers that claimed to explain this situation, in incompatible ways.* So they can't be both true.** It demonstrates that at least one of them is measuring something wrong. This is an extremely common problem in the social sciences. Once there is an established mystery, many people propose mechanisms to explain the discrepancy. They go out and measure and many find that the effect is just about the right size. It never happens that five people each explain 10% of the gap.

* One paper made a point of surveying prostitutes. The other used lie detectors. But the details are not important.

** Conceivably both effect sizes could be true, if there is yet another effect that cancels one out. Age difference is an effect in the opposite direction. But it is extremely unlikely that the three effects are all the same size.

Honestly, from the things I've heard, setting gays at ten percent of the population, the idea that gay men have 20 times as many partners as heterosexuals or lesbians isn't that hard to imagine.

See also previous research into this phenomenon like http://www.physorg.com/news10824.html and http://www.mv.helsinki.fi/home/jproos/jonasdottir.htm

There is also a question of whether they are using "average" to refer to mean or median.

With standards like that, the other statistics in the article are probably also untrustworthy. "I do wonder under what other misapprehensions I continue to labour."

The median vs average comment sums it up. Imagine a society with one female prostitute who services all of the otherwise monogamous and mate-for-life males. In this society, the median for women is one, the median for men is two. Similarly, if we average while discarding the outliers we will see an asymmetric average.

I expect there's a way to get more accurate survey results using a randomized response method (which I learned about from this post). I can't quite figure out how you'd do it, though.

You seem to be missing a more obvious explanation here, even if you assume 100% heterosexuality. Robin Hanson refers to it as hypergamy. According to the theory, the most desirable males have lots of female mates, frequently including a lot of cuckolding of less desirable males during the females' estrus.

Editing to add, since I decided this wasn't clear: A simple model of this situation would have many men with 1 sexual partner, many women with 2 sexual partners (husband + highly desirable lover), and a few men with a huge number of sexual partners.

ETA: Eh, never mind, the math still doesn't work.

Easy answer. Men forget who their partners were and double count. Plus women who are dumped frequently have reconstructive surgery in order to have a second chance. :P