People in western countries are more promiscuous, the study says

Monogamy is dominant across the world, but multiple partners are more common in rich countries, according to the study published in the Lancet.

This was despite developing countries having higher rates of sexually transmitted infections and HIV.

London School of Hygiene and Tropical Medicine researchers gathered data from 59 countries for the study.

They said factors such as poverty and mobility had more of a role in sexually transmitted infections than promiscuity had.

But the team added that the findings showed teenagers were not having sex earlier, contrary to popular beliefs.

The study said there had been no universal trend towards earlier sexual intercourse over the past three decades.

Almost everywhere, sexual activity began for most men and women between 15 and 19 years of age, with men tending to start earlier.

In the UK the average age for men was 16.5 and for women 17.5.

Researchers said most people reported only having one sexual partner in the last year.

However, those reporting multiple partners were much higher in developed countries - up to a third of under 25s in some areas - whereas only a small percentage in Africa reported the same.

And among singletons, westerners were more sexually active as well.

Two thirds of men and women without a partner in African countries reported they had had sex recently, compared to three quarters of those in developed countries.

Surprising

The researchers said these findings were surprising, as higher rates of STIs were reported in developing countries.

But the report's author, Professor Kaye Wellings, said: "This suggests social factors such as poverty, mobility and gender equality may be a stronger factor in sexual ill-health than promiscuity."

And she added that the results showed flexible approaches to tackling public health had to be adopted.

"Men and women have sex for different reasons and in different ways in different settings," she said.

"This diversity needs to be respected in a range of approaches tailored to whole societies, and to particular groups and individuals within them.

"The selection of public-health messages needs to be guided by epidemiological evidence rather than by myths and moral stances."

A spokeswoman for Brook, the sexual health charity for young people, added: "There is still much to be done to improve sexual health in this country.

"But the situation here does not compare to that of many developing countries where it can be very difficult to get condoms and to insist on their use, for a range of economic and cultural reasons."