The parents of children with autism are less likely to go on to have more children than the parents of children without the condition, a new study has found.

Autism, also known as autism spectrum disorder (ASD), is a biological disorder of the brain that impairs communication and social skills. Signs can include an absence of eye contact, apparent aloofness, avoidance of physical contact even with family, difficulty dealing with interruptions to routine and a lack of interest in other children and what they are doing.

"While it has been postulated that parents who have a child with ASD may be reluctant to have more children, this is first time that anyone has analysed the question with hard numbers," commented Prof Neil Risch of the University of California in San Francisco (UCSF).

The researchers decided to look into the issue of ‘reproductive stoppage' - when parents make a conscious decision not to have further children - after having a child with autism.

They identified over 19,700 families which included a child with ASD. These were then compared to over 36,000 families with no history of ASD, who acted as controls for the study.

The researchers found that overall, families whose first child had ASD were a third less likely to go on to have a second child compared to the control families. If a child with ASD was not the first-born, these families were equally less likely to go on to have more children.

In others words, a significant number of parents chose not to have any more children after having a child with ASD, whether this was their first child or not.

The study noted that the decision of whether to have more children did not appear to be affected until the child with ASD started to display symptoms or was diagnosed. This suggests that the decision not to have more children was down to the choice of the parent, rather than a fertility problem.

"This study is the first to provide convincing statistical evidence that reproductive stoppage exists...Our work shows that not only do people with ASD have fewer children than others, but in families where a child has ASD, the fact that the parents choose to have fewer children means the genes that predispose to ASD are less likely to be passed on to future generations," Prof Risch said.

However, this finding raises another query, which has not yet been figured out.

"ASD has an important genetic component, which should be diminishing over time due to this reduction in childbearing. Yet over the past several decades, the incidence of ASD has risen dramatically," Prof Risch noted.

Details of these findings are published in the journal, JAMA Psychiatry.