U.S. scientists have identified a gene variant that makes some mothers get excessively angry with their children.

A new study has found that that difficult economic conditions may lead to mothers being excessively harsh with their children, including shouting at them or slapping them.

It found that mothers with a particular gene variation are more prone to losing their temper with their children too easily during an economic downturn.

The study revealed women who carry a variant of the protein are predisposed to hitting or shouting at children during a difficult economic climate.

The 'harsh parenting' phenomenon was found to have increased during the 'Great Recession' in the U.S. between 2007 and 2009, particularly among women with the particular gene variant.

Just over half the parents taking part in the study had the gene known as DRD2, that controls dopamine, which regulates a person's behaviour and mood.

The research was based on data from the Fragile Families and Child Wellbeing Study (FFS), involving nearly 5,000 children born in 20 large American cities between 1998 and 2000.

Mothers were interviewed shortly after giving birth and when the child was about one, three, five and nine years old.

The results showed that mothers with the genetic variant were much more likely harshly patent their children where there were deteriorating local economic conditions and a decline in consumer confidence.

The research was carried at New York University, Columbia University, Princeton University and Pennsylvania State University's College of Medicine, and published in the journal Proceedings of the National Academy of Sciences.