Tonight the group finds out about the powerful evolutionary forces shaping sexual attraction. The women make some uncomfortable discoveries tonight, when they realise how universal their sexual tastes are: it seems they can be turned on by virtually any image portraying sexual activity. Under the guidance of Dr Erick Janssen of the Kinsey Institute, the women learn that their arousal to such diverse stimuli may serve the important evolutionary purpose of preparing the body for the prospect of any sort of harmful penetration. Later, 10 strapping men arrive at the lab and the group takes part in some entertaining experiments designed for the series by evolutionary psychologist Dr John Manning, to find out why they are attracted to certain men and not others. With one of the girls' boyfriends secretly infiltrating the line-up, there's a lot at stake: can she pick her own boyfriend out of a line-up of anonymous attractive men? As usual the girls share their experiences, discuss the results of the experiments and quiz the scientists on their work and what it means for women.