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Biting, Hitting, Kicking — The Developmental Trajectory of Physical Aggression

By Leon MoriguchiPublished May 13, 2026Updated May 18, 20266 min read日本語版あり
Audience
Parents of children ages 1–4

TL;DR

  • ·Physical aggression peaks around age two and declines in the great majority of children; humans appear to be born with aggression tendencies and reduce them through socialization, not the reverse
  • ·The slope of the decline matters more than the peak level — watching whether aggression is less than six months ago is more informative than comparing against a single-point average
  • ·A roughly 5–14% "chronically high" group whose aggression does not decline warrants professional attention; language delay combined with high-frequency aggression is one of the clearer signals for early support

Contents

  1. Lead
  2. Physical Aggression Is a Declining Phenomenon
  3. The Roughly 5–10% Whose Aggression Does Not Decline
  4. Relational Aggression: A Separate Developmental Track
  5. Where Does Intervention Become Appropriate?
  6. Recording Creates a Trajectory, Not Just an Event
  7. Summary
  8. References

Lead

One afternoon, a daycare calls: your child bit another child. The following week, they hit someone. At home, they took a toy from a younger sibling and, when the sibling resisted, kicked them.

In that moment, certain questions probably cross your mind. Is this abnormal? Is our parenting to blame? Are we raising a violent child?

The short answer, based on decades of developmental research, is that biting, hitting, and kicking around age two are well within the typical range. Declining from that peak is the norm. The real question is not "Is my child aggressive?" but "How is this behavior changing over time?" This article traces what longitudinal data show about the trajectory of physical aggression and identifies the threshold that warrants closer attention.

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