Is he always looking for the worst, cheapest and dirtiest restaurant? Does he eat a lot only when he receives an invitation? The roses are always a little withered, aren't they? Don't put the blame on him, but on his genes.

An international research team has found for the first time that genes influence the way people act in the 'economic game'. Traditionally, behavior scientists have avoided linking the economic behavior to the genes.

But the new research made by David Cesarini, a Ph.D. student in MIT's Department of Economics and Sweden researchers have found the opposite: there is a genetic side when it comes to people's perception on what is fair and what not.

The subjects were tested with the ultimatum game, in which a proposer comes with an offer to a responder on how to share a money amount. The offer is an ultimatum; if rejected, both players lose the money. As rejections mean nobody gains anything, theoretically, narrow self-interest says that whatever the positive sum, it will be accepted by the responder.

But responders routinely reject the proposals, perhaps for punishing purposes, as the offers are considered unfair. To see how genes influence the responders' behavior, the team recruited twins from the Swedish Twin Registry and the volunteers were tested with the game.

As identical twins have the same genes (while fraternal twins do not), the genetic influences could be observed by comparing the way identical and fraternal twins behaved during the game.

The genetic influences appeared to have a role in up to 40 % of the variation in people's reaction to unfair offers. Identical twins were found to adopt the same strategy than fraternal twins.

"Compared to common environmental influences such as upbringing, genetic influences appear to be a much more important source of variation in how people play the game," said Cesarini.

"This raises the intriguing possibility that many of our preferences and personal economic choices are subject to substantial genetic influence," said lead author Bjorn Wallace of the Stockholm School of Economics.