The plus team in Plus Magazine:
Straight men have had twice as many sexual partners, on average, as straight women. Sounds plausible, seeing that men supposedly think about sex every seven seconds. Except that it's mathematically impossible: in a closed population with as many men as women (which roughly there are) the averages should match up. Someone is being dishonest, but who? And why? These questions, along with many others, are explored in Sex by numbers, a new book by David Spiegelhalter, Winton Professor for the Public Understanding of Risk at the University of Cambridge.
“Sex is a great topic,” says Spiegelhalter. “There's lots of it going on, but we don't know what goes on or how much of it, because most of the time it goes on behind closed doors. It's a really difficult topic to investigate scientifically, and a real challenge for statistics.” Spiegelhalter's aim is to get people interested in a critical approach to the numbers they hear about in the news and give them the tools to figure out if they can be believed. “It's really a book about statistics, using sex as an example.”
Statistics about sex are not all equally good. Some, like the number of births in a given year, are cast-iron facts, but others are much harder to come by. The number of sexual partners is a good example. The mismatch above comes from the third The National Survey of Sexual Attitudes and Lifestyles (Natsal), conducted between 2010 and 2012, in which men reported having had 14 sexual partners, on average, and women 7. Studies have suggested that women give lower numbers when they fear the survey isn't entirely confidential, something that doesn't seem to affect men (contrary to my expectation, it doesn't induce them to exaggerate). So that's one possible explanation for the mismatch: sadly, women still need to fear social stigma.
But there are other explanations too. One is that men (more than women) may have some of their sexual experience with sex workers. These aren't included in the surveys, so their experiences are missing from the female tally. Another is that there are different attitudes as to what counts as a sexual partner. If a woman feels she's been coerced by a man, for example, she may not want to count him.
More here.