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Sibling Age Gap — What the Evidence Actually Says

By Leon MoriguchiPublished May 15, 2026Updated May 18, 20266 min read日本語版あり
Audience
Parents considering a second child, or parents of siblings

TL;DR

  • ·The medical evidence supports an interpregnancy interval floor of at least 18 months to reduce perinatal risks — "two years" is a rough approximation of that threshold, not a precise target
  • ·Economics research finds wider spacing modestly benefits the older child's cognitive outcomes, but no specific gap is identified as optimal, and birth-order effects are largely independent of spacing
  • ·For outcomes parents care about most — sibling relationship quality and closeness over time — the evidence base is thin, and those judgments belong to the family

Contents

  1. Lead
  2. The Economics Angle: Spacing and Test Scores
  3. The Perinatal Medicine Angle: Maternal Recovery and Infant Health
  4. Unpacking the "Two-Year" Folk Theory
  5. What the Evidence Can and Cannot Say
  6. Summary
  7. References

Lead

"Two years apart is ideal." "Three years apart and they won't fight." "Irish twins are exhausting, but you get it all over with." The opinions on sibling spacing are plentiful — from your own parents, from the waiting room at the pediatrician, from every parenting book that dares to take a position.

With this much folklore on the table, it's easy to assume that a correct answer exists. What does the evidence actually say? Researchers in economics, developmental psychology, and perinatal medicine have each approached sibling spacing from a different angle, and the picture they produce together is considerably more complicated than "two years is best."

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