Researchers at the University of Maryland tracked 84 infants from birth to age 2, with about one-third characterized as highly irritable and two-thirds characterized as moderately irritable.
Irritability was measured using a test administered in the home within a month of the babies' births. The infants had to react to a series of events, including being undressed and hearing a bell ringing.
When the children were 18 and 24 months, they were observed in a laboratory setting to assess how they responded to being around unfamiliar adults and toys.
The researchers measured infants' attachment at 12 months, based on the babies' behavior when observed with their mothers -- securely attached infants turn toward the mother when distressed and use her for comfort; insecurely attached infants do not, the researchers say.
The study, published in the journal Child Development, finds highly irritable newborns were the most sociable as toddlers if they were securely attached and the least sociable as toddlers if they were insecurely attached.
In addition, highly irritable infants who were insecurely attached were the least able to engage in exploration as toddlers. However, the quality of infants' attachment was not related to either exploration or sociability in toddlers who were moderately irritable as newborns, the study says.