Previous RAND research also showed that teens who watch a lot of television with sexual content are more likely to initiate intercourse the following year.

CNN: Study links sexual content on TV to teen pregnancy

Early marriage in many part of Sub-Saharan Africa and Asia means that many engage in intercourse even before they are teenagers and have limited choice over birth control.

BBC: Educating girls is vital to a country's economy, Plan says

Another, in 2001, not specifically of rape, found that among 221 women trying to conceive, "one completely random act of unprotected intercourse" had a 3.1% chance of ending in pregnancy.

WSJ: Pregnancies From Rape Prove Tough to Count

In Renaissance Italy, it turns out, there was no end of advice as to how to get them: would-be parents of sons should eschew strong drink, endure abstention to build up their strength and then have intercourse early in the day.

ECONOMIST: Renaissance advice books

"Let us then, fellow citizens, unite with one heart and one mind, let us restore to social intercourse that harmony and affection without which liberty, and even life itself, are but dreary things, " he said in a classic expression of our great tradition of conciliation.

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The US Food and Drug Administration (FDA) approved Levitra, an orange pill compared to Viagra's blue, based on studies showing that men were on average five times more likely to achieve an erection suitable for intercourse when taking the pill compared with those given a dummy medicine.

BBC: Viagra pills

After reviewing data on how married couples in the U.S. tackle housework, as well as self-reports of how often they enjoyed intercourse, sociologists at the University of Washington (UW) say that couples who shared the burden of chores -- cooking, cleaning and caring for the lawn -- tend to have the least active sex lives.

CNN: Why husbands who share household chores miss out on sex