Over at Certain Doubts, Gregory Wheeler reviews our lies about sex:

In survey after survey within country after country men report having more heterosexual partners over their lifetime than women do, and as this article and this clarification point out, what people say in these surveys cannot be a reflection of what they do.

If these surveys were representative, we would expect that the average number of heterosexual partners reported by men in each sample to approximate the average number reported by women. Instead, the numbers aren’t even close. In Britain men report having an average of 12.7 hetrosexual partners over a lifetime whereas women report an average of 6.5; in France men report an average of 11.6 heterosexual partners, women 4.4; and in Germany men say 15.5, women 10.1.  …

But the interesting feature of Brown’s study was that he also asked the respondents to rate the truthfulness of their estimates. He found that 5% of men and 4% of women indicated that they thought their estimates were inaccurate, and 16% of men and 11% of women indicated that they knowingly misrepresented their counts. Still, even when these "self-incriminators" were removed from the sample population, there was still a significant discrepancy between the counts for men and women.

Yes, many are aware they lie about sex.  But it seems many others are not aware.  That suggests that you, yes you, do not really know how many sexual partners you have had!

Added: The movie Secrets and Lies has an ambiguous case: does she lie or self-deceive?