['In previous research, low resting heart rate in childhood and adolescence has been shown to predict aggressive and/or delinquent behavior at subsequent ages. It has been found that heart rate recorded as early as age 3 years could predict externalizing behavior at age 11 years. This study explored the possibility of a similar relationship between heart rate and externalizing behavior problems.', 'Heart rate recorded at ages 14, 20, 24, 36 months and 7 years was used to predict combined parental ratings on the Aggressive and Delinquent Behavior scales of the Child Behavior Checklist (CBCL/4-18) as well as the Externalizing composite scale measured at age 7 years. Subjects consisted of same-sex twin pairs, treated as singletons in the present study, participating in the MacArthur Longitudinal Twin Study. Subjects were grouped into high and low heart rate groups and also into high CBCL/4-18 scoring and low CBCL/4-18 scoring groups.', 'Heart rate was not significantly related to scores on either of the 2 subscales or the Externalizing composite scale at any age. Heart rate group membership did not predict CBCL/4-18 scores. Conversely, CBCL/4-18 group membership did not predict heart rate at any age.'] Does early childhood heart rate predict externalizing behavior problems at age 7 years?