Feuding siblings the world over may now have the science to back up what they've long suspected... parents really do have a favourite child.
Research published in the Journal of Family Psychology has revealed that 74 per cent of mothers and 70 per cent of fathers reported preferential treatment toward one child.
However, parents did not let on which child they preferred.
For the study, sociologist Katherine Conger and her research team followed 384 families where a pair of siblings was born within four years of each other.
The scientists thought they had a clear hypothesis on who in the family would feel the most hard done by when it came to brownie points from the folks, but the result proved them wrong.
"Our working hypothesis was that older, earlier born children would be more affected by perceptions of differential treatment due to their status as older child - more power due to age and size, more time with parents - in the family," Conger told news site Quartz.
However, the firstborns often reported feeling like they were the preferred child, while the younger siblings said they could sense a firstborn bias, which affected their self-esteem much more than the older children.
Firstborns tended to feel preferred as their status in the pecking order made them the first in the family to score in sports, lead the way academically, and generally confound their parents as to what to do, according to NY Magazine's Science of Us.
Eldest children led by example, and when younger kids get to the age of their older siblings, parents had a better idea of what to expect and tended to get a little tougher - at least, that's one theory.
Perhaps unsurprisingly, the research also found that everyone, regardless of birth order, harbours a sneaking suspicion that they are being treated unfairly.
"Everyone feels their brother or sister is getting a better deal," Conger said. "Regardless of how you look at it, both [earlier and later-born kids] are perceiving preferential treatment."
While sibling rivalry seems like an inescapable part of life, the news isn't all bad. A study from the University of Toronto found having an older sibling can help to boost your intelligence, while research from Ohio State University says growing g up with siblings may make you less likely to get a divorce as an adult.