RESEARCH_FILE
The Science of Sibling-in-Law Rivalry
Marrying into a family doesn't create a blank slate — it plugs a new person into a scorekeeping system that started decades earlier, in the nursery.
SEE THE PRACTICE
Turn a thought this research explains into one clear move.
THE THOUGHT
“We're the failure couple of the family”
YOUR RECORDED RESPONSE
“I didn’t agree to this comparison game.”
ONE PRIVATE MOVE
On a blank note card, write three columns: “theirs,” “ours,” and “not mine to score.” Fill in one item for each, then fold the card and place it in your wallet or a drawer before the day is over.
Research on sibling rivalry, going back to the 1980s, shows that comparison and perceived favoritism between siblings start in early childhood and persist across the lifespan. Separately, research on in-laws shows why affinal relatives — people related by marriage, not blood — are structurally more prone to friction than genetic kin. Put the two bodies of research together and 'sibling-in-law rivalry' looks less like personal pettiness and more like two well-studied systems, family favoritism and kinship conflict, colliding at a wedding. Here's what decades of sibling and in-law research actually shows.
How the science changed
- 1982
Judy Dunn and Carol Kendrick publish Siblings: Love, Envy, and Understanding, the first systematic observational study of siblings from the birth of a second child. It documents that rivalry and jealousy are a normal, near-universal reaction, not a sign of family dysfunction, and that they are shaped by how differently parents treat each child from the very start. ↗
- 1997
Martin Daly, Catherine Salmon, and Margo Wilson argue in 'Kinship: The Conceptual Hole in Psychological Studies of Social Cognition and Close Relationships' that relationships forged by marriage follow different rules than relationships forged by blood — in-laws don't automatically share the same genetic stake in each other's welfare that siblings do. ↗
- 2013
Gilligan, Suitor, Kim, and Pillemer study 341 adult children nested in 137 later-life families and find that a father's — but not a mother's — current favoritism predicts tension between adult siblings, especially among daughters. ↗
- 2013
Jensen, Whiteman, Fingerman, and Birditt show in '"Life Still Isn't Fair"' that young adults continue to perceive differential parental treatment well into their twenties and thirties — the childhood sense that a parent plays favorites doesn't end with childhood. ↗
- 2021
Daly and Perry's evolutionary review of in-law relationships formalizes why they run 'good, bad, and ugly': in-laws are recruited allies rather than automatic ones, and rivalry over a shared relative's finite time, attention, and resources is a predictable structural feature — not a personal failing. ↗
- 2022
Jensen, Apsley, Thackeray, and Shoaf publish the first study of sibling social comparison in mid- to later life, surveying 491 adults and finding that comparing yourself to a sibling is linked to both more depressive symptoms and conflict — and, at the same time, greater closeness. ↗
- 2025
Jensen and Jorgensen-Wells's meta-analysis of nearly 19,500 participants across dozens of studies confirms parental favoritism is a real, measurable, patterned phenomenon — parents on average favor daughters, more conscientious children, and (for autonomy and control) older siblings. ↗
What people believe vs. what the data shows
The belief“Sibling rivalry is something you outgrow once you're an adult with your own family.”
The dataYoung adults in their twenties and thirties still report perceiving differential parental treatment, and comparison with siblings continues to be measurable — and to affect wellbeing — into mid- and later life. ↗
The belief“If your spouse's siblings compete with or exclude you, it means something is specifically wrong with you.”
The dataEvolutionary kinship theory treats friction between in-laws as a predictable structural feature of blended kin systems — affinal relatives don't automatically share the same stake in each other's welfare that blood relatives do, so rivalry over a shared relative's attention is a system-level pattern, not proof of a personal flaw. ↗
The belief“Parents don't really have favorites — that's just how it feels to the less-favored child.”
The dataA 2025 meta-analysis of nearly 19,500 participants across dozens of studies confirms differential parental treatment is real, measurable, and patterned — by the child's gender, birth order, and personality — not a distortion of perception. ↗
The belief“Comparing yourself to your sibling's spouse or your spouse's siblings is petty and has nothing to do with real family psychology.”
The dataSocial comparison between family members is a documented, measurable behavior studied well into mid- and later life, and it's linked to real outcomes — both more conflict and depressive symptoms, and, in the same data, greater closeness. ↗
The belief“Whether someone who married in 'really' counts as family is just etiquette, not something psychology studies.”
The dataQualitative family research shows people do active, ongoing boundary-work deciding who counts as 'real' family, and that in-laws' inclusion or exclusion from that personal definition is tied to the actual quality of the relationship, not just politeness. ↗
TEST_YOURSELF · How well do you know this science?
01 What did Dunn and Kendrick's foundational 1982 observational study conclude about sibling rivalry?
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 ↗
02 In Gilligan, Suitor, Kim, and Pillemer's 2013 study of later-life families, whose perceived favoritism predicted tension between adult siblings?
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 ↗
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?
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 ↗
04 The 2022 study of sibling social comparison in mid- to later life found that comparing yourself to a sibling was linked to:
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 ↗
05 The 2025 meta-analysis of nearly 19,500 participants (Jensen & Jorgensen-Wells) found that, on average, parents were more likely to favor:
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 ↗