Turns out, the perception of favouritism may have more effect on a child-parent relationship than was previously considered.
Specifically, Brigham Young University's Alex Jensen found that favouritism is linked more to younger siblings' parent-child relationships than with the older siblings'.
If the younger sibling feels like they're the favourite, and the parents agree, their relationship is strengthened. If they don't feel like the favourite and the parents agree with that, the opposite happens. Surprisingly with older siblings, whether they feel favoured or not, it has no major impact on the relationship.
What could be the reason behind this difference? Jensen said that the social comparison - one sibling comparing himself to the other - is the culprit.
"It's not that first-borns don't ever think about their siblings and themselves in reference to them," Jensen said. "It's just not as active of a part of their daily life. My guess is it's probably rarer that parents will say to an older sibling, 'Why can't you be more like your younger sibling?' It's more likely to happen the other way around."
The data in the study were collected from a longitudinal study with more than 300 families, each with two teenage children.
To measure levels of favouritism, researchers looked at responses from both the children and their parents. The children were asked what their relationship with their parents is like while their parents were asked how much warmth and conflict they experienced with their children. They found that children, on average, have more warmth and more conflict with their mothers, but the rates of change in relationship for both mother and father were similar.
The study looked at families with two children, but Jensen believes that the data would show similar results for larger families as well.
While parents may think treating their children equally is the best way to mitigate any negative effects, Jensen says this is not the case.
"When parents are more loving and they're more supportive and consistent with all of the kids, the favouritism tends to not matter as much," Jensen said. "Some parents feel like 'I need to treat them the same.' What I would say is 'No you need to treat them fairly, but not equally.' If you focus on it being okay to treat them differently because they're different people and have different needs, that's OK."
The study appears in Journal of Adolescence.