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

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.

Physical Aggression Is a Declining Phenomenon

For most of the twentieth century, the prevailing assumption was that aggression in young children is learned and accumulates — amplified by television, poor discipline, and environment.

A series of longitudinal studies by Richard Tremblay and colleagues overturned that assumption. Tremblay's group followed 572 infants from five months of age, assessing maternal reports of physical aggression at 17, 30, and 42 months [1]. The finding was striking: by 17 months, the large majority of children were already showing some physical aggression toward siblings, peers, or adults [1]. Behaviors such as hitting, pushing, and grabbing peak between ages two and four, then decline gradually in most children [1,2].

The implication is a fundamental reframing: rather than learning to become more aggressive, humans are born with a tendency toward physical aggression and reduce it through socialization [2]. A toddler who bites or hits is not defective — inhibitory control simply has not yet arrived.

Large-scale longitudinal datasets from Canada and the United States, integrated in a meta-analytic framework, replicate the same pattern: a clear peak around 24 months and a pronounced decline toward school entry [3].

The Roughly 5–10% Whose Aggression Does Not Decline

Here is the important fork in the road. In Tremblay's cohort study of children followed from 17 to 42 months, trajectory analysis identified three groups [1]:

A low-aggression group showing almost no physical aggression (~28%)
A moderate group whose aggression declined over time (the large majority)
A "chronically high" group that remained at high frequency and did not decline (~14%)

Nagin and Tremblay's well-known longitudinal study — following 1,037 boys in Montreal from ages 6 to 15 — found similar differentiation [4]. Four trajectories emerged: a chronic high group, a high-but-declining group, a moderate-declining group, and a near-absent group. The chronic group represented only a few percent of the sample, but their subsequent risk for violent delinquency was substantially higher than the other three groups [4].

The takeaway is direct: what parents should watch is not "Is my child aggressive?" but "Is it decreasing with age?" The slope of the trajectory matters more than the absolute level at any single point [1,4].

Relational Aggression: A Separate Developmental Track

Once physical aggression subsides, something else sometimes takes its place: . This concept was articulated by Crick and Grotpeter in 1995 to describe behaviors that harm others through social means — excluding peers, spreading negative talk, using the silent treatment [5].

In their study of 491 school-age children, Crick and Grotpeter found that relational aggression was a dimension independent of physical aggression, more prevalent among girls, and associated — in both perpetrators and victims — with loneliness, depression, and peer rejection [5].

A longitudinal study by Côté and colleagues (2007) followed 1,183 children from age two to eight and modeled the joint trajectories of physical and indirect (relational) aggression [6]. Physical aggression declined with age, while indirect aggression increased from around age four onward. The two trajectories formed different combinations across children [6].

This matters because "stopped hitting" does not automatically signal maturity. Verbal and social forms of harm — subtler, and more tolerated by adults — may be taking the place of fists. Recognizing this shift is different from treating it as equally urgent; it is simply a different developmental challenge that deserves its own attention later.

Where Does Intervention Become Appropriate?

"Peak physical aggression at age two is normal" and "intervention is warranted" are two separate claims that can both be true.

Tremblay's research group has consistently argued for early intervention with the chronically high-aggression group [2,4]. Situations worth considering for professional consultation include:

Physical aggression that has not meaningfully decreased by ages three to four
Repeated incidents causing injury to other children
Language delay that has left aggression as the primary means of communicating needs (the relationship between language development and physical aggression is well documented in the toddler period [7])
Persistent interpersonal difficulties severe enough to disrupt group participation at daycare or preschool

Conversely, if a two-year-old is hitting and biting frequently but vocabulary is growing, rules are beginning to register, and the frequency has been falling over a six-to-twelve month window, those are signs that the learning curve is working as expected [1,2].

Among things families can do at home, the evidence most consistently supports expanding the child's repertoire of alternatives without removing the function of the aggressive behavior. If biting reliably gets the desired toy, the behavior is reinforced. If the child discovers that verbalizing "I want that" also works — and works consistently — language becomes an alternative route [7]. Whether and how to respond to the act matters less than whether a different channel exists and gets results.

The connection between language development and physical aggression runs in both directions. Dionne and colleagues (2003) studied 19-month-old twins and found that expressive vocabulary and physical aggression were negatively correlated at that age: children with a richer productive vocabulary showed lower rates of physical aggression, and this relationship held even when shared genetic background [7]. Language does not merely label feelings after the fact — it provides a functional substitute for physical action. This is why language delay, in combination with high-frequency aggression, is one of the clearer signals that a child may benefit from earlier professional support rather than a wait-and-see approach.

Recording Creates a Trajectory, Not Just an Event

Seeing a trajectory requires a line, not a point. Pulling out a single incident — they bit someone today — leaves a parent perpetually exhausted. Being able to compare this month to last month, or this season to six months ago, is what makes "is the arc bending downward?" a question you can actually answer.

A tool like Memori, which preserves daily events in sequence, suits this kind of observation. Logging "bit at daycare" or "hit sibling" is not about compiling evidence for someone else — it is so that you, six months later, can read back and ask: was that the peak?

And when you read that log six months on, you might find: "March — called in to daycare." Alongside it: "September — used words to ask, didn't grab." The event becomes readable only when it carries change with it.

Summary

Physical aggression peaks around age two and declines in the great majority of children [1,2]. The slope of that decline is more predictively important than the peak level itself [1,4]. A small group — roughly 5–10% — shows a chronic high trajectory that does not decline, and this group warrants closer monitoring and, in some cases, early professional involvement [4]. Alongside the decline in physical aggression, relational aggression may emerge as a separate and later-developing form of harm, distinct from its physical counterpart [5,6].

The question to ask is not "Is my child aggressive?" but "Is it less than six months ago?" If the answer is yes, that child is doing the work of learning to be less physically aggressive. That is not nothing.

References

Tremblay RE, Nagin DS, Séguin JR, Zoccolillo M, Zelazo PD, Boivin M, et al. Physical aggression during early childhood: trajectories and predictors. Pediatrics. 2004;114(1):e43–e50. doi:10.1542/peds.114.1.e43. PMID: 15231972.
Tremblay RE, Vitaro F, Côté SM. Developmental Origins of Chronic Physical Aggression: A Bio-Psycho-Social Model for the Next Generation of Preventive Interventions. Annu Rev Psychol. 2018;69:383–407. doi:10.1146/annurev-psych-010416-044030. PMID: 29035692.
Tremblay RE. The development of aggressive behaviour during childhood: What have we learned in the past century? Int J Behav Dev. 2000;24(2):129–141. doi:10.1080/016502500383232.
Nagin D, Tremblay RE. Trajectories of boys' physical aggression, opposition, and hyperactivity on the path to physically violent and nonviolent juvenile delinquency. Child Dev. 1999;70(5):1181–1196. doi:10.1111/1467-8624.00086.
Crick NR, Grotpeter JK. Relational aggression, gender, and social-psychological adjustment. Child Dev. 1995;66(3):710–722. doi:10.1111/j.1467-8624.1995.tb00900.x. PMID: 7789197.
Côté SM, Vaillancourt T, Barker ED, Nagin D, Tremblay RE. The joint development of physical and indirect aggression: Predictors of continuity and change during childhood. Dev Psychopathol. 2007;19(1):37–55. doi:10.1017/S0954579407070034. PMID: 17241483.
Dionne G, Tremblay RE, Boivin M, Laplante D, Pérusse D. Physical aggression and expressive vocabulary in 19-month-old twins. Dev Psychol. 2003;39(2):261–273. doi:10.1037/0012-1649.39.2.261. PMID: 12661885.