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    The Arizona Sibling Study was supported by grant DA06287 from the National Institute on Drug Abuse. We thank Marilyn King for her oversight of data collection and analysis and Travis Hirschi for providing comments on an earlier draft of this paper.

  • David C. Rowe is Professor, Family Studies and Genetics, at the University of Arizona. He has over 60 publications in the child development, personality, and behavior genetic literature. His research interests include the behavior genetics of personality traits and behavioral deviance.

  • Bill L. Gulley is a graduate student in Family Studies at the University of Arizona. He is expert in multivariate analysis and statistics. His major research interests include adolescent delinquency.


Sibling effects refer to the immediate influence one sibling may have on another or to indirect influences through their embeddedness in a common friendship network We used three aspects of sibling mutual interaction—warmth, conflict, and frequency of contact with mutual friends—to evaluate sibling effects on delinquency and substance use in 135 brother pairs, 142 sister pairs, and 141 mixed-sex pairs in the Arizona Sibling Study (primarily aged 10–16 years). We hypothesized that sibling relationship variables would condition the behavioral resemblance of the younger and older sibling. For both substance use and delinquency, this prediction was confirmed for warmth and mutual friends: Sibling pairs who reported warmer mutual relationships or greater contact with mutual friends were more alike behaviorally. The statistical sibling effects were not explained by social class, parental substance use, or rearing styles. We interpret them as the influence of one sibling on the other and as the influence arising from sharing common friends. Given the existence of sibling effects, the strength of shared familial influences of other origins must be revised downward.