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

Key findings

1. 1982

   Observing families from the birth of a second child, Dunn and Kendrick found rivalry and jealousy to be a normal, near-universal reaction, strongly shaped by how differently parents treated each child from the very start. ↗

2. 1992

   Reviewing the accumulated evidence, Dunn concluded that the quality of the sibling relationship — how much warmth versus rivalry it carries — shows continuity from early childhood and is linked to children's broader social understanding, not just to how they get along at home. ↗

Key findings

1. 2013

   With Gilligan, Kim, and Pillemer, studying 341 adult children in 137 later-life families, Suitor found a father's — but not a mother's — current favoritism predicted tension between adult siblings, especially among daughters. ↗

2. 2013

   Tracking mothers' favoritism over time, Suitor and colleagues found it to be remarkably stable across years — a mother's 'favorite' rarely changes — meaning perceived favoritism is a durable feature of the family system, not a passing phase. ↗

3. 2016

   Modeling data on 381 older mothers and their 1,421 adult children, Suitor and colleagues found that how similar a child's values are to the mother's — more than the child's achievements — was the strongest predictor of maternal favoritism and disfavoritism. ↗

Key findings

1. 2013

   With Whiteman, Fingerman, and Birditt, Jensen showed young adults continue to perceive differential parental treatment well into their twenties and thirties — the sense that 'life isn't fair' between siblings doesn't end with childhood. ↗

2. 2022

   With Apsley, Thackeray, and Shoaf, Jensen surveyed 491 adults and produced the first study of sibling social comparison in mid- to later life, finding it linked to both more depressive symptoms/conflict and, in the same data, greater sibling closeness. ↗

3. 2025

   With Jorgensen-Wells, Jensen meta-analyzed nearly 19,500 participants across dozens of studies and confirmed parental favoritism is real and patterned — parents on average favor daughters, more conscientious children, and, on measures of autonomy and control, older siblings. ↗

Key findings

1. 2021

   With Gretchen Perry, Daly reviewed evidence that affinal kin (in-laws) share zero genetic relatedness with each other yet both have a stake in the same descendants — a structural asymmetry that makes conflicts of interest more likely than among blood relatives. ↗

2. 2021

   The review flags the mother-in-law/daughter-in-law dyad as carrying the highest structural conflict potential of any in-law pair across the cultures surveyed — a cross-cultural pattern the authors attribute to competing investment interests, not individual character. ↗