Widespread use of the antibiotic penicillin during the 1950s and not the development of contraceptives may have ushered in the sexual revolution in the US, a new study has found.

The rise in risky, non-traditional sexual relations that marked the swinging 1960s actually began as much as a decade earlier, during the conformist 1950s, suggests an analysis recently published by the journal Archives of Sexual Behaviour.

"It's a common assumption that the sexual revolution began with the permissive attitudes of the 1960s and the development of contraceptives like the birth control pill," said Emory University economist Andrew Francis, who conducted the analysis.

"The evidence, however, strongly indicates that the widespread use of penicillin, leading to a rapid decline in syphilis during the 1950s, is what launched the modern sexual era," Francis said.

As penicillin drove down the cost of having risky sex, the population started having more of it, Francis said, comparing the phenomena to the economic law of demand: When the cost of a good falls, people buy more of the good.

"People don't generally think of sexual behaviour in economic terms but it's important to do so because sexual behaviour, just like other behaviours, responds to incentives," he said.

Syphilis reached its peak in the US in 1939, when it killed 20,000 people.

"It was the AIDS of the late 1930s and early 1940s. Fear of catching syphilis and dying of it loomed large," Francis said. Penicillin was discovered in 1928, but it was not put into clinical use until 1941.

"The military wanted to rid the troops of STDs and all kinds of infections, so that they could keep fighting. That really sped up the development of penicillin as an antibiotic," he said.

Right after the war, penicillin became a clinical staple for the general population as well. In the US, syphilis went from a chronic, debilitating and potentially fatal disease to one that could be cured with a single dose of medicine.

From 1947 to 1957, the syphilis death rate fell by 75 per cent and the syphilis incidence rate fell by 95 per cent.

"As soon as syphilis bottoms out, in the mid- to late-1950s, you start to see dramatic increases in all three measures of risky sexual behaviour," Francis said.