Physical reasons such as to reduce stress ("It seemed like good exercise"), feel pleasure ("It's exciting"), improve or expand experiences ("I was curious about sex"), and the physical desirability of their partner ("The person was a good dancer").

Goal-based reasons, including utilitarian or practical considerations ("I wanted to have a baby"), social status ("I wanted to be popular") and revenge ("I wanted to give someone else a sexually transmitted disease").

Emotional reasons such as love and commitment ("I wanted to feel connected") and expression ("I wanted to say 'thank you'").

Insecurity-based reasons, including self-esteem ("I wanted the attention"), a feeling of duty or pressure ("My partner kept insisting") and to guard a mate ("I wanted to keep my partner from straying").

Researchers analyzed responses from two sets of married or cohabitating couples: one group was together for one to three years, the other for four to six years.

While the researchers could not pinpoint a precise turning point - the seven-year itch, as popularized in the play and film about errant husbands, was largely a theory - they found distinct differences between the groups.

"We know the earlier ones are happier," said Prof. Kelly Musick, a University of Southern California sociologist. "The initial boost that marriage seems to provide fades over time."

Research also showed that the median duration of first marriages that end in divorce remains a little more than seven years, which means that those couples will likely spend more than half their married lives less happy than they were when they cut the first slice of wedding cake.

"Some folks start getting less happy at the wedding reception," said Larry Bumpass, a professor at the University of Wisconsin-Madison, who wrote the study with Professor Musick.

In a survey of 1,000 British adults by car hire firm Holiday Autos 58 per cent of Brits admitted getting it on in their motors, with the flipped passenger seat position the most popular.