London, Mar 18 (IANS): If your 10-year-old is becoming aggressive day-by-day, blame the hormone cortisol for the increased aggression in the young boy, says a new study.

Findings by Spanish researchers confirm that children, especially boys, who experienced the greatest increase in levels of aggression by ten years of age were those whose cortisol levels had also increased during eight-to-ten years of age.

The research team studied the relationship between hormones and aggressive behaviour in girls and boys between the ages of eight and ten.

They measured the levels of three steroid hormones -- testosterone, estradiol and cortisol -- and analysed the effects of hormones on behaviour in the age group.

Researchers analysed 90 children -- 49 boys and 41 girls -- from four primary schools.

The results demonstrated that there was indeed a change in the levels of aggressive behaviour.

"This only occurred in boys: at ten years of age they were more aggressive than at eight years of age. The girls, nevertheless, did not experience any changes to their levels of aggression during these two years," said Eider Pascual-Sagastizabal from the University of the Basque Country in Northern Spain.

The researchers found that the boys whose levels of aggression had increased most by the age of ten were those whose levels of cortisol had also increased over the two years.

On the contrary, the boys whose levels of aggression had decreased most between eight and ten years of age were the ones whose levels of estradiol had increased most between the two ages.

"A greater increase in cortisol is linked to a greater increase in levels of aggressive behaviour, while a greater increase in estradiol corresponds to a decrease in levels of aggressive behaviour", adds Pascual-Sagastizabal.