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Why are children in the same family so different from one another?

  • Robert Plomin (a1) and Denise Daniels (a2)

One of the most important findings that has emerged from human behavioral genetics involves the environment rather than heredity, providing the best available evidence for the importance of environmental influences on personality, psychopathology, and cognition. The research also converges on the remarkable conclusion that these environmental influences make two children in the same family as different from one another as are pairs of children selected randomly from the population.

The theme of the target article is that environmental differences between children in the same family (called “nonshared environment”) represent the major source of environmental variance for personality, psychopathology, and cognitive abilities. One example of the evidence that supports this conclusion involves correlations for pairs of adopted children reared in the same family from early in life. Because these children share family environment but not heredity, their correlation directly estimates the importance of shared family environment. For most psychological characteristics, correlations for adoptive “siblings” hover near zero, which implies that the relevant environmental influences are not shared by children in the same family. Although it has been thought that cognitive abilities represent an exception to this rule, recent data suggest that environmental variance that affects IQ is also of the nonshared variety after adolescence.

The article has three goals: (1) To describe quantitative genetic methods and research that lead to the conclusion that nonshared environment is responsible for most environmental variation relevant to psychological development, (2) to discuss specific nonshared environmental influences that have been studied to date, and (3) to consider relationships between nonshared environmental influences and behavioral differences between children in the same family. The reason for presenting this article in BBS is to draw attention to the far-reaching implications of finding that psychologically relevant environmental influences make children in a family different from, not similar to, one another.

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J. F. Dunn & C. Kendrick (1982) Siblings: Love, envy, and understanding. Grant McIntyre. [aRP]

H. J. Eysenck & M. W. Eysenck (1985) Personality and individual differences: A natural science approach. Plenum. [MZ]

J. Kagan & H. A. Moss (1962) From birth to maturity. Wiley. [PTC]

H. Lytton (1980) Parent-child interaction: The socialization process observed in twin and singleton families. Plenum. [SS, TDW]

A. Tellegen , D. T. Lykken , T. J. Bouchard , K. Wilcox , N. Segal & S. Rich (in press) Personality similarity in twins reared apart and together. Journal of Personality and Social Psychology. [DTL]

T. D. Wachs & G. Gruen (1982) Early experience and human development. Plenum. [TDW]

M. Zuckerman , D. M. Kuhlman & Camac , (in press) What lies beyond E and N? Factor analyses of scales believed to measure basic dimensions of personality. Journal of Personality and Social Psychology. [MZ]

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Behavioral and Brain Sciences
  • ISSN: 0140-525X
  • EISSN: 1469-1825
  • URL: /core/journals/behavioral-and-brain-sciences
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