University of California researchers quizzed 384 families and found that 74 per cent of mothers and 70 per cent of fathers had a definite favourite among their children.
Although the parents were not asked to name their favourite child, follow-up questioning of their children revealed eldest children were preferred by the vast majority.
The study questioned pairs of teenage siblings no more than four years apart, on how they feel their parents treat them.
A sociological survey asked if they sensed a differential treatment between their siblings, and how that has affected their confidence.
The results showed a clear bias towards first-borns that affected the confidence of younger siblings, damaging their self-esteem.
The results came as quite a shock to university study leader Katherine Conger, who says the research had been aiming to prove the opposite – that younger children were often the favourites.
It’s a common-held belief among most feuding siblings that the youngest always gets all the attention and the most love.
“I was a little surprised,” Katherine admits. “Our hypothesis was that older, earlier-born children would be more affected by perceptions of differential treatment due to their status as the older child in the family.”
A previous study from the University of Toronto discovered having an older sibling could help to boost intelligence.
Separate research from Ohio State University said growing up with brothers or sisters may also make divorce less likely as an adult.