Stress can lead to arguments
Stressed parents may play a role in childhood asthma, researchers believe.
They found the children of tense parents who lived in polluted areas were far more likely to have asthma than friends in the same neighbourhood.
The University of Southern California team believe parental anxieties combine with other known risk factors to increase a child's asthma risk.
They told Proceedings of the National Academy of Sciences there might be an underlying biological explanation.
Experts have already shown that women who are stressed in pregnancy may raise the risk of their child developing asthma or other allergies.
And stress is known to trigger asthma attacks.
In the latest study the researchers followed 2,497 healthy primary school children living in Southern California and recorded how many of these developed asthma over a three-year period - 120 in total.
They also gathered information on other known asthma risk factors like exposure to traffic-related air pollution and maternal smoking, as well as parental education, income and stress levels.
As expected, children exposed to more air pollution had a higher risk of asthma, but this risk was further increased if their parents were stressed and described their lives as "unpredictable", "uncontrollable" or "overwhelming".
Maternal smoking and parental stress posed a similar compounded risk.
Professor Rob McConnell and his team speculate that stress increases the inflammatory effects of pollutants in tobacco smoke and traffic fumes on the airways.
Writing in PNAS they said: "These results suggest that children from stressful households are more susceptible to the effects of traffic-related pollution and in utero tobacco smoke on the development of asthma."
Elaine Vickers of Asthma UK said: "This study adds to existing evidence suggesting that a child's environment can impact on their risk of developing asthma.
"For example, smoking during pregnancy, traffic pollution and stress in the home may all have harmful effects.
"We know that smoking during pregnancy significantly increases a baby's risk of having breathing difficulties and that children whose parents smoke are 1.5 times more likely to develop asthma, so Asthma UK strongly advises parents to avoid smoking around children and young people, especially in the home.
"One in 11 children in the UK has asthma so studies like this are vital, as they provide an insight into the factors influencing asthma development and therefore how it might be prevented."