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 suggest this behaviour is normal. 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. 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. Also addressed in this final installment of the series, does the 'gaydar' really exist?