This is the research companion to Why Does She Get To?. Read the main post first.
The main post argued that the uneven deals in a family with more than one kid are unavoidable, and that what children seem to react to is how they read those deals. Here's what that rests on. One large review sizes the link between unequal treatment and kids' wellbeing. One small study shows how much a child's sense of fairness changes the picture. A second review puts unequal treatment in proportion next to sibling conflict and warmth. One looks at the siblings of autistic children, and one draws the line between ordinary rivalry and bullying. Most of these samples are Western and mostly white, which is worth keeping in mind throughout.
The Size of It
Jensen, A. C., & Thomsen, A. E. (2024). Parental differential treatment of siblings linked with internalizing and externalizing behavior: A meta-analysis. Child Development, 95(4), 1384-1405.
DOI: 10.1111/cdev.14091
What they found: A preregistered meta-analysis of 43 samples, 26,451 children and 2,890 effect sizes found that being treated differently from a sibling was linked with more internalizing problems (worry, sadness, withdrawal) and more externalizing problems (acting out). The links were small, with correlations of roughly .07 to .11, and they were stronger when children reported the differential treatment than when parents did.
Why this matters for you: It sizes the worry. Unequal treatment counts, and it's a modest part of the picture. The stronger link for children's own reports is what points the main post toward the child's read of the deal.
What it doesn't answer: It's correlational, so it can't say which way things run; children who are struggling may draw different treatment, not just respond to it. The samples were 82 percent White and 55 percent from the US, and the analysis can't separate differences children see as justified from ones they don't.
Fair Changes the Picture
Kowal, A. K., Krull, J. L., & Kramer, L. (2004). How the differential treatment of siblings is linked with parent-child relationship quality. Journal of Family Psychology, 18(4), 658-665.
DOI: 10.1037/0893-3200.18.4.658
What they found: In 74 two-parent families with adolescent siblings, teens rated how differently their parents treated them and whether the differences were fair. When teens judged a parent's differential affection unfair, it was linked with a more hostile, less warm relationship with that parent; when they judged it fair, the link was essentially zero. Differential control (rules and discipline) was linked with more hostile, less warm relationships in teens' reports whether or not they judged it fair.
Why this matters for you: This is the finding behind "fair, and explained." For affection at least, the child's judgment of fairness made the difference between a link and no link.
What it doesn't answer: It's small, cross-sectional and homogeneous (98 percent white, intact families). The fairness effect was clean for affection only. Crucially, it doesn't test whether a parent explaining the reasons changes how fair a child judges things to be; that's the main post's inference, and we've labeled it as one.
Proportion
Buist, K. L., Deković, M., & Prinzie, P. (2013). Sibling relationship quality and psychopathology of children and adolescents: A meta-analysis. Clinical Psychology Review, 33(1), 97-106.
DOI: 10.1016/j.cpr.2012.10.007
What they found: Pooling 34 studies and 12,257 children and teens, sibling conflict was linked with internalizing (r = .27) and externalizing problems (r = .28), roughly twice the size of the links for sibling warmth (r = -.12 and -.14) and for parental differential treatment (r = .14 and .18). The differential-treatment link with internalizing was stronger for children than for adolescents.
Why this matters for you: Proportion. Unequal treatment matters, and how siblings treat each other matters more, which is a good reason not to let worry about perfect fairness eat all the available energy.
What it doesn't answer: The studies are correlational, and the authors say directly that causal and directional claims aren't warranted. Samples were entirely Western and mostly white, and the warmth-internalizing estimate didn't hold up to a check for unpublished studies. It can't say where ordinary squabbling turns into the kind of chronic conflict that carries the risk.
Where the Cost Shows Up
Shivers, C. M., Jackson, J. B., & McGregor, C. M. (2019). Functioning among typically developing siblings of individuals with autism spectrum disorder: A meta-analysis. Clinical Child and Family Psychology Review, 22(2), 172-196.
DOI: 10.1007/s10567-018-0269-2
What they found: Across 69 samples, siblings of autistic individuals showed a small overall difference from comparison groups (g = -0.26). The differences clustered inward, in depression, anxiety, beliefs about disability and a less positive sibling relationship (the largest area, g = -0.42), with no difference in externalizing behavior, adjustment, coping, family functioning or self-concept. Parent and self-reports agreed.
Why this matters for you: It's the pattern behind the main post's check-in. When there's a cost, it tends to show up as worry and low mood rather than as behavior anyone has to deal with, which makes it easy to miss.
What it doesn't answer: These are group comparisons, and family differences rather than autism itself may explain part of them. Results varied widely between studies, and a correction for unpublished studies cut the overall estimate in half (to g = -0.13). It doesn't cover siblings of children with ADHD or other conditions, and averages aren't destiny for any child.
The Line
Bowes, L., Wolke, D., Joinson, C., Lereya, S. T., & Lewis, G. (2014). Sibling bullying and risk of depression, anxiety, and self-harm: A prospective cohort study. Pediatrics, 134(4), e1032-e1039.
DOI: 10.1542/peds.2014-0832
What they found: In a UK birth cohort, children answered questions about sibling bullying at 12 and were assessed by clinical interview at 18. Those bullied by a sibling several times a week had higher rates of depression (12.3 percent compared with 6.4 percent among those not bullied). After adjustment for other family and child factors, the odds were about 1.85 times higher for depression and 2.26 times higher for self-harm. The link with anxiety didn't hold up after adjustment.
Why this matters for you: It's the evidence for drawing a clear line. Uneven deals are ordinary; one child persistently bullying another is a different thing, with outcomes that look more like peer bullying than like ordinary sibling friction.
What it doesn't answer: It's observational, the bullying was self-reported at a single point, and families who stayed in the study were more advantaged than those who left. Odds ratios describe groups, not what will happen to any one child.
Coming Up
The hour before bed, and why the evening starts going well or badly long before the lights go out.
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