"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.

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.

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


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