Jul 27 2005
If men thought they were frittering away money wining and dining a girl to win her hand, they should think again.
Dr Peter Sozou and Professor Robert Seymour from University College London (UCL) have developed a mathematical model that shows how expensive but worthless gifts may help facilitate courtship.
Reporting in the journal Proceedings of the Royal Society of London B they analysed the function of a courtship gift and what the characteristics of a 'good' gift are.
They show that gifts can act as a signal of a man's intention. Offering an expensive gift may signal a long-term commitment but the man must be wary of being exploited by a gold-digger who intends to dump him once she gets the gift.
By modelling courtship as a sequential game, they show that an extravagant gift, which is costly to the man but worthless to the woman, may solve the problem.
A costly gift signals the man has long-term intentions but by being worthless to the woman, gold-diggers are deterred.
The researchers also show that a modified form of the model may apply to species where males do not help to raise the young.
Dr Peter Sozou, of UCL's Centre for Mathematics and Physics in the Life Sciences and Experimental Biology, says:
"Gift-giving by males is a feature of human courtship and mating systems in a number of species. Females invest more resources than males in offspring and so must take care to pick the best partner possible, something that's not always easy to gauge from general cues such as appearance.
"In humans, a girl wants a guy who is attractive to her and will help raise their children. The worst pay-off, reproductively, is if she hooks up with an unattractive male who, literary, leaves her holding the baby.
"Guys are less likely to offer expensive gifts to females they don't have a long-term interest in. And girls won't be impressed by cheap gifts. By offering expensive but worthless gifts, such as dinners and theatre trips, the male pays no cost if the invitation isn't accepted. Girls that don't find a guy attractive are less likely to take up the invitation because it would mean spending time with a person they aren't interested in.
"In other species the deciding factors for a female is whether she's in a sexually receptive state and the male is in a good condition. Males offer gifts which may signal their condition. Those in a poor condition can't offer the same quality of gift."
The researchers constructed two versions of the game with different biological assumptions based on whether the male is involved with parental care.
Attractiveness is relevant to human courtship (model 1). Male condition and female receptiveness are the deciding factors in non-parental care species (model 2). In both cases these were represented as binary variables.
Factors in the game such as whether the male and female found each other attractive were given a probability and the possible outcomes of the interaction, either positive or negative for each player, were given scores to represent the consequences of their decisions.
Professor Robert Seymour, of UCL's Department of Mathematics, says:
"We assumed that a male's expected pay-off from mating with a female is positive. But it's greater if he finds her attractive, and then it's worth him staying around after mating – so we'd give it a high score.
"Conversely, if he finds her unattractive, his post mating pay-off is maximised by deserting, giving him a lower score.
"These scenarios are represented mathematically and analysed to find each player's best strategy."
They considered the 'fitness' consequences based on a single courtship encounter involving a male and female. Despite the different biological assumptions, the two models had the same underlying mathematical structure, with both yielding equilibrium solutions in which males predominantly offer costly but worthless gifts as a prelude to mating.
"Our analysis shows that there is evolutionary logic in men 'burning money' to impress the girl," added Professor Seymour.