Series 1 Episode 3
First Broadcast: 12AM Thu 30 June 2011

The sex researchers have spent a good deal of time watching, recording and measuring our sexual activities to find out what is normal.

If you're a man, in a long-term relationship but cheating on your partner, the answer seems to be that this behaviour is normal. Men's propensity to stray from the marital bed seems to be encoded into their genes.

One researcher even claims to have found the cheating gene. Another offers up even more radical evidence, claiming that the shape of a man's penis is proof positive that we've evolved to be unfaithful.

The ultimate sexual survey was carried out by legendary sexologist Alfred Kinsey. He exposed the sexual antics of America and helped facilitate the sexual revolution of the 60s.

In fact it's the work of the sex researchers that has paved the way for the sexual tolerance of the 21st century. In the western world homosexuality in particular has moved from being a criminal offence to an accepted part of human sexuality.

And the sex researcher's focus has shifted from trying to find out if gay men are normal to trying to find out why they're different. One recent study is aiming to work out if it really is possible to pick out a gay man in a crowd; does the 'gaydar' really exist?

Even though the scientific research into cheating men seems depressingly predictable there is good news for any romantics out there.

Sex researchers now believe that we have not one but three competing sex drives: one is the good old fashioned sex drive (which is often prone to stray); the second is love, which unleashes a torrent of hormones, turns our lives upside down and persuades us to start a relationship with someone; and the third is attachment, which keeps us together.

These three drives are not just social constructs but hard-wired into our brains and bodies. It's why, no matter what our sexual preference, monogamy is, in fact, normal.

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For over 100 years, pioneering men and women have been uncovering our deepest secrets. Their methods have been visionary, kinky and sometimes bizarre, and their findings have transformed our sex lives.