If you are planning to add yet another new member to your family, think again! A new study has found that children in larger families are more likely to fall behind in cognitive achievement and have behavioural and other problems.

"Families face a substantial quantity-quality trade-off: increases in family size decrease parental investment, decrease childhood performance on cognitive tests and measures of social behaviour," the study said.

"Importantly, we find that these negative effects are not merely temporary disruptions following a birth but in fact persist throughout childhood," the researchers wrote.

"A lot of what happens in early childhood has lasting impact," said one of the study authors Chinhui Juhn, professor at the University of Houston in the US.

"In many respects, this matters more than a lot of things that happen later in (a child's) life," Juhn noted.

The study used a dataset that tracked outcomes throughout childhood and compared outcomes of older children before and after a younger sibling was born.

They found that additional children reduce "parental investment," a category defined as including time spent with children, affection, the safety of the home environment and resources - money, books and other material goods.

"If you are in a well-resourced family, some of these things do not apply," Juhn said.

"When the second child comes along, there is less time and attention. But in an environment with more resources, it is not as binding," Juhn pointed out.

The study appeared in a paper of National Bureau of Economic Research, a research organisation.