The babies, part of the the Millennium Cohort Study, were assessed for their temperaments when they were 9 months old, using a validated scale, designed to pick up positive mood, receptivity to new things, and sleep and eating patterns -- regularity -- in infants.

The York University researchers classified the mothers as either non-smokers during pregnancy, quitters, light smokers, or heavy smokers, those who smoked 10 or more cigarettes a day.

The study, published in the the Journal of Epidemiology and Community Health, found the mothers who quit smoking had the most easy going infants, compared with non-smokers and smokers.

These children had the lowest chances of unpredictable behavior and of becoming distressed when faced with new situations or things, while heavy smokers had the most difficult children, who scored the lowest for positive mood.

Previous animal research has also shown that nicotine is a behavioral toxin, the study said.