The Science of Sibling-in-Law Rivalry: Why the Old Family Scoreboard Never Really Closes

TEST_YOURSELF · How well do you know this science?

1. 01 What did Dunn and Kendrick's foundational 1982 observational study conclude about sibling rivalry?

   It only appears in families with three or more childrenIt's a normal, near-universal reaction shaped by differential parental treatment from the startIt disappears completely once the older child starts schoolIt is caused primarily by birth order alone

   Dunn and Kendrick's observational study, starting from the birth of a second child, found rivalry and jealousy to be a normal, near-universal reaction shaped by how differently parents treated each child from the very beginning — not a sign of a uniquely dysfunctional family. source ↗

2. 02 In Gilligan, Suitor, Kim, and Pillemer's 2013 study of later-life families, whose perceived favoritism predicted tension between adult siblings?

   The mother's favoritismThe father's favoritismGrandparents' favoritismNeither parent's favoritism predicted tension

   The study found a father's — but not a mother's — current favoritism predicted tension between adult siblings, with the effect especially pronounced among daughters. source ↗

3. 03 According to Daly and Perry's 2021 evolutionary review, why do in-law relationships tend to generate more friction than relationships between blood relatives?

   In-laws share no automatic genetic stake in each other's welfare, so their interests aren't guaranteed to alignIn-laws are legally required to share resources equallyCultural taboos universally forbid closeness between in-lawsIn-law conflict is a purely modern phenomenon absent in earlier human societies

   Daly and Perry, building on earlier work on kinship theory, argue in-laws are 'recruited' rather than automatic allies: unlike blood relatives, they don't automatically share the same genetic stake in each other's welfare, making rivalry over a shared relative's attention and resources a predictable structural pattern. source ↗

4. 04 The 2022 study of sibling social comparison in mid- to later life found that comparing yourself to a sibling was linked to:

   Only negative outcomes, without exceptionOnly positive outcomes, without exceptionBoth more depressive symptoms and conflict, and greater closeness, in the same dataNo measurable effect on wellbeing at all

   Jensen, Apsley, Thackeray, and Shoaf found sibling social comparison in mid- to later life linked to both more depressive symptoms and more conflict, as well as, in the same sample, greater closeness — a genuinely mixed-outcome pattern rather than a purely negative one. source ↗

5. 05 The 2025 meta-analysis of nearly 19,500 participants (Jensen & Jorgensen-Wells) found that, on average, parents were more likely to favor:

   Sons over daughtersDaughters over sonsWhichever child moved out of the house firstNo child consistently — the meta-analysis found favoritism to be a myth

   Across the pooled studies, parents on average favored daughters, along with more conscientious and agreeable children; on measures of autonomy and control specifically, they favored older siblings. The analysis concluded favoritism is real, not a myth. source ↗