PARIS: Differences between males and females in behaviour are usually chalked up to sex-specific hormones, but a study released shows that genes play a critical role too.

Experiments at Yale University with mice revealed that sex chromosomes alone are enough to account for the fact that females, for example, are more likely to be creatures of habit.

The results, published in Nature on Sunday, have important implications for understanding the origins of addiction, and could one day lead to treatments targeting the genes involved. "This is the first time that any behaviour has been associated specifically with sex chromosomes independent of gonadal hormones," the study's lead author, Jennifer Quinn of Yale University, said.

Scientists have long observed that female mammals-whether four-legged ones that squeak or two-legged one that talk -tend more than males toward habit-forming behaviour, including addiction. Sex-specific hormones regulated by gonadal organs explained part of the difference, but not all.

To find out whether genes also played a role in the Mars-Venus divide, a team of scientists led by Yale's Jane Taylor devised an elaborate set of experiments involving made-to-order mutant mice.

Through genetic manipulation and breeding, the researchers came up with two mice variants in addition to normal males and females: one mouse with male gonads but female sex chromosomes, and another with female gonads and male sex chromosomes. This made it possible to measure the impact of hormones and genes separately.

In the experiments, mice in each of the four groups figured out on their own which of three side-by-side openings led to food. What started out as goal-directed behaviour -the search for something to eat -rapidly became habit: within days, all the mice scampered straight to the correct door without even hesitating.