Not since the 18th century has there been such a shift in human behaviour. That century saw a loosening of religious ties and sexual mores, accompanied by a new importance placed on individual happiness. Philosophers such as Jean Jacques Rousseau, Voltaire, Thomas Paine and Jeremy Bentham stressed the importance of the individual's rights.

The desire for freedom and equality fuelled the desire to avoid marriages of commerce and convenience and the rise of the love match. And it lent a kind of personal morality to the extramarital affair. So a marquess who had entered into a dynastic marriage might believe he deserved to be happy with a lover (or lovers) of his choice. His dalliance might involve intrigue and betrayal, but it was all done in the name of love.

The social historian Dr Julie Peakman points out that the expansion of London in the 18th century made the city a magnet for those leaving the country in search of work. Then the world of work was opening up for women, allowing them economic independence and freedom. "Women weren't just confined to being barmaids and seamstresses," Peakman says. "They ran the new coffee houses and brewed ale. They entered Grub Street and ran the new printing presses that churned out the popular scandal sheets." Many of these women felt free enough to take lovers. They could meet them in Ranelagh or Vauxhall, two of the capital's open-air pleasure gardens, or leave a note for an assignation at their dressmaker.

When it came to sex, adultery was mainly confined to those with money and opportunity: the aristocracy and the emerging merchant class. Married women could have affairs, but they must be discreet. A strict code evolved where decorum and secrecy were paramount. An academic who has studied these nuances is Dr Frances Wilson, a lecturer at Reading University. She cites the example of two famous adulteresses of the Regency period: Emma Hamilton, the mistress of Lord Nelson, and Lady Caroline Lamb, who dallied with Lord Byron. "Emma behaved with decorum, so society accepted her relationship with Nelson," says Wilson. "By contrast, Lady Caroline Lamb was a bit of a Glenn Close bunny-boiler." When Byron ended the affair she refused to leave him alone and followed him everywhere, finally slashing her wrists in front of him at a ball. She survived, but was ostracised by society. When she walked into a room it would empty.

Modern women engaged in affairs can readily identify with one of the most famous adulteresses of the 19th-century novel, Madame Bovary, who takes her own life. Such women found their lives complicated, compromised and impossible. Today's women, perhaps, find theirs just as complicated and compromised — but possible.

Lascivious Bodies, by Julie Peakman, is published by Atlantic Books