Violence between parents can reduce young children's IQ levels, researchers say.
Surveys suggest the problem is widespread
A study of 1,116 pairs of five-year-old twins in the UK suggests that in homes where mothers are abused by their partners, the children's IQs are on average eight points lower than usual.
The researchers say this is the equivalent of a three-inch reduction in physical height - and even if not a lasting effect, is happening just as the children start school.
The study was carried out by researchers at King's College London in the UK and Boston University in the United States.
They studied identical and fraternal twins to minimise the likelihood that the children with lower IQs had simply inherited them.
In the study, children whose mothers reported no domestic violence had IQs of about 100, the average in the general population.
Those exposed to high levels of domestic violence had IQs that were eight points lower on average - a "moderately strong" effect.
Karestan Koenen, of the department of child and adolescent psychiatry at Boston, said: "This finding is important because low IQ at age five is a risk factor for negative outcomes such as school failure, and later, juvenile delinquency."
Terrie Moffitt of the institute of psychiatry at King's said: "Eight IQ points may not sound like very much, but it is equivalent to three inches on the height chart for five year olds.
"If domestic violence stunted children's physical growth by three inches, doctors would take that seriously.
"We do not know if this effect is temporary or lasting, but we do know that it is present at the age when children begin school, and their mental growth is very important at that age."
Prof Moffitt said the effect put the children about six months behind their peers in terms of their development.
She said it had been known for a few years that - surprisingly - environmental factors including levels of stress could affect the development of intelligence.
Their current study would also be looking at such factors as how much children were read to by their mothers and what range of stimulating toys they had available, but it had begun with the violence issue.
This accounted for probably about 10% of the environmental impact on IQ.
As to why it had an effect, that was not clear, but it could be indirect as well as a direct result of having witnessed violence.
"It could have been for example that depression in the mother due to violence meant she failed to care for the children properly."