Aggression Is It Innate
This study examines the prevalence, stability, and development of physical aggression, as reported by mothers
and fathers, in a sample of children initially recruited at 12, 24, and 36 months (N52,253) and in a subsample
followed up 1 year later (n5271) in a cross-sequential design. Physical aggression occurred in 12-month-olds,
but significantly more often in 24- and 36-month-olds. The rates of physically aggressive behaviors increased in
the 2nd year of life, and declined from the 3rd birthday onward. Stabilities were moderate for 12-month-olds
and high for 24- and 36-month-olds. At the ages of 24 and 36 months, boys were more aggressive than girls. The
results confirm and extend R.E. Tremblay’s (2004) hypothesis about the early development of physical
aggression.
Recent studies indicate that the frequency of children’s
physical aggression is highest during the
preschool years (e.g., NICHD, 2004; Tremblay et al.,
2004). It has been suggested that physically aggressive
behaviors, such as hitting, kicking, and biting,
occur as early as around the first birthday, increase
and peak during the 2nd and 3rd year of life, and
decline in the 4th year of life (Cummings, Ianotti, &
Zahn-Waxler, 1989; Gimenez & Blatier, 2004;
Tremblay et al., 1999, 2004). However, the few studies
that have investigated the development of aggression
rates in early childhood did not include
data regarding all relevant age groups (i.e., each of
the first 4 years of life). In addition, sex differences in
the rate of aggressive behavior in these age groups
have been largely ignored. Thus far, it is unclear at
what age the frequently reported sex differences in
aggression (see Coie & Dodge, 1998) start to emerge.
The present study addresses these issues in a general
population sample of 1- to 4-year-old boys...
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