Longitudinal observational data of infant crying, fretting/fussing, and smi
ling and the time spent in physical contact with the mother were used in a
study on behavioral variability. The infants were followed weekly for a 15-
month period. Evidence was found of an important intraindividual variabilit
y in the studied behaviors, specifically between the ages of 0 and 5 months
, 5 and 10 months, but not between 10 and 15 months. Results are discussed
and analyzed in the light of earlier literature. The possible functions and
the factors underlying variability in development as well as its methodolo
gical implications are discussed.