McConnell and colleagues used data from 2,497 children aged five to nine years, who were taking part in the USC Children's Health Study, investigating respiratory health among children in 13 southern California communities.

None of the children had a history of asthma or wheezing at the start of the study

(2002-2003). The researchers followed them for three years and noted any parental reports of doctor-diagnosed new onset asthma in their children during that time.

Stress was also linked to a larger effect from being exposed to tobacco smoke while in the uterus.

These findings were published in the early online issue of the Proceedings of the National Academy of Sciences.