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

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