From The Independent:
The book explores the role played by smell in physical attraction (men can tell by the scent, even though they are not aware of it, when a woman is ovulating); analyses the significance of the wording of lonely hearts advertisements; examines the strange phenomenon of religious love for an invisible God; and weighs up the rival benefits, in terms of gene propagation, of males adopting the strategies of either monogamy or philandering.
Dunbar's quest is to find out why we evolved into a (generally) monogamous species; and the answer he turns up is unsettling. It appears that women are the choosers in our species, and they choose on the basis of which male is likely to offer the best protection for their offspring.
“Infanticide by males is a perennial risk for monkeys and apes,” says Dunbar, because killing a female's young offspring stops her lactating and makes her fertile again. Our distant female ancestors were therefore more likely to choose males as “hired guns” who would keep them and their children safe from the predations of other males. For this reason, women generally have more reason to maintain the pairbond than men do. It's painful to reflect that the splendour of love has such grisly origins.