'Emotionally, they make plans and have strategies, while men are more impulsive.' From a young age, he says, women will start to consider the kind of life they want, what kind of job or house they aspire to, how many children they'd like and what kind of partner.
'Most men don't really develop that facility until they're much older.
They live, psychologically, in the moment, while women think around corners and two steps ahead.' We're also better at mental multi-tasking.
'You can keep all the different strands of a lie in your minds, remember them and make them convincing.' Men lie, too, but they are much more likely to forget something, or make a mistake and get caught out.
If we meet in the evening, I tell Adrian I'm with my sister, and meet Mark a few miles away, where we won't bump into anyone.' Does she feel guilty about the affair? You know the 11th commandment: don't get caught.' Women have always had affairs, but over the past 20 years that number has risen dramatically.
Jobs outside the home - with the ready-made excuse of working late or business travel - financial independence and changing social attitudes mean that modern women simply have more opportunity to meet other men and start affairs.
According to Dr David Holmes, a psychologist at Manchester Metropolitan University, women are having more affairs than ever - recent studies say the figure is around 20 per cent for men and a bit over 15 per cent for women - but they behave very differently from men when they cheat.
'The biggest difference is that women are much better at keeping their affairs secret,' he says.
When studies about sexual partners or fidelity use a mixture of face-to-face interviews and anonymous computer questionnaires, men will give the same answers to both, but women will report much higher numbers when the answers are anonymous. Can you imagine a leading female politician having an affair and her husband standing loyally by her?
Or the reaction to a female CEO having a public affair with her young male assistant?
When men have affairs, he says, they tend to be bigger risk-takers and naïve about how obvious their cheating is to an emotionally astute partner.
They will also be bursting to tell someone about it. They will often want to brag about it, or be so taken up by the sexual thrill of the affair that their behaviour is a giveaway.' Women, on the other hand, are much more cool and rational, even when they're in the grip of passion. They may confide in one or two very close women friends, but they compartmentalise their emotional lives and don't let the affair bleed into the rest of their lives.' Like Farrah Fawcett, a former colleague of mine, Susan, now 50, had a long-term affair with Brian, a salesman she met, ironically enough, when she was buying her husband a car as a surprise 40th birthday present.
The truth is that we have always lied about our sex lives.