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sibling cohort study

A large-scale study drawing on sibling cohorts has identified associations between birth order and the likelihood of diagnosis across a range of conditions, including attention-deficit hyperactivity disorder and autism spectrum diagnoses. The scale of the dataset gives the findings more statistical weight than smaller studies on similar questions, but the results are associations, not proof of a causal mechanism, and researchers involved in this area of work have been careful to frame them that way even as headlines tend to compress the nuance.

• The study is observational, meaning it identifies statistical associations rather than proven causal mechanisms.
• Detection bias — parents and clinicians recognising symptoms differently based on prior experience with older siblings — is a leading candidate explanation for some patterns.
• Findings varied across the specific conditions studied, indicating no single unifying mechanism is likely to explain all of them.
• Sibling-cohort designs reduce some confounding factors but cannot fully rule out others, including factors specific to pregnancy order or parental age at each birth.