They were as different as can be imagined. The older brother, only 18 months senior, was much quieter and more structured and disciplined, had fewer friends, and seemed content to be alone. The younger brother was far more spontaneous, expressive of his feelings, affectionate, and had a larger circle of friends. These differences were obvious from early childhood and seemed to account for their different developmental trajectories. The older brother was by far the better student and went on to graduate school. In contrast, the younger brother spent only 2 years in college, leaving it to take a job in sales. At midlife, the older brother had a stable professional career and marriage while the younger brother had gone from one sales job to another and was in his third marriage.
Not surprisingly, their parents and extended family believed that these differences in personality were entirely inherited. The differences were apparent so early in life that it was widely held that their subsequent developmental outcomes flowed directly from their different genetic endowments. After all, what difference could there be in their environment? They were raised by the same parents, lived in the same neighborhoods, went to the same schools, and had the same opportunities.
This formulationsibling differences as a result of only genetic differenceshas been shown to be a simplistic and misleading reductionism. What is surprising, perhaps, is that the challenge to this reductionism has come, to a considerable extent, from research aimed at studying how differences in genetic makeup account for differences in siblings' personalities.
Quantitative behavioral genetics relies on a methodology in which the personalities (and development of psychiatric disorders) are compared in siblings of different degrees of genetic relatedness (eg, twins or adopted siblings) raised together or apart. A large number of studies have demonstrated that sibling differences arise from 3 categories of variables: genetics, shared environmental influences, and non-shared environmental factors. In addition to documenting the role of genetic factors in personality, these studies have consistently indicated that non-shared environmental factors play a major role in producing personality differences in siblings.Non-shared environmental factors
Non-shared environmental factors are multiple and include differential parental treatment, gender, birth rank order, different experiences by siblings of the sibling relationship itself, different peer influences, and the occurrence of such nonsystematic events as illnesses, accidents, and the like. Some of these nonshared environmental factors interact with each other in complex ways. Our own research, for example, suggests that in more functional families, sibling rank order is very influential, whereas in more dysfunctional families, gender is more decisive. We concluded that the influences of sibling rank order and gender differed as a function of family competence. 1 Emphasis must be placed, however, on the fact that these nonshared environmental factors are assumed to be interwoven with genetic differences; siblings, to a significant extent, do create some of the environmental responses through the influence of partially genetically determined personality characteristics.
The attempt to document the exact nature of non-shared environmental factors has proved to be most difficult. For example, David Reiss, the noted family researcher, and his colleagues have reported the results of a monumental longitudinal study of adolescent development that by its sheer scope, surpasses most other developmental studies.2 More than 700 families containing 2 same-sex siblings who were 4 or less years apart were studied on 2 occasions separated by a 3-year interval. With use of the procedures of quantitative behavioral genetics, the 2 adolescent siblings were categorized according to their different degrees of genetic relatedness (from monozygotic twins to adopted siblings who have no genetic relatedness). What is most impressive about this study, however, is that environmental influencesat least those operating within the familywere directly observed and measured, using state-of-the-art interactional techniques. Thus, this research project was carefully designed to elicit information about non-shared environmental influences (as well as genetic and shared environmental influences).
The study's results add much to our understanding of adolescent development: the important role of genetic influences was documented and the earlier results from quantitative behavioral genetic research that suggested a modest role, if any, for shared environmental influences were contradicted. Shared environmental influences (eg, stable family characteristics) do account for a significant part of adolescent personality in crucial areas such as autonomy and sociability.Internal representations
Even in this remarkable study, however, the nature of the non-shared environmental influencesa very significant contributor to personality differencesremains elusive. The data supplied do not inform about what these factors are. Reiss discusses this surprising turn of events in a careful manner and makes a cogent case that the answer may lie in the psychoanalytic concept of internal representations of self and others.
In other words, the manner in which experiences are taken into the self, stored, recalled, and narrated is believed to be a highly individualistic process. Two siblings could internalize the same experiences within the family or in their social world in very different ways, and these differences in representations could play important roles in personality development and consequent sibling differences. Reiss also discusses the fact that although techniques have been developed to identify internal representations, his study's methodology (a sample of more than 700 families and 1400 adolescents) did not include the opportunity to conduct the intensive interviews or carry out the other measures necessary to identify internal representations.
This step, however, has recently been described in another remarkable longitudinal study. Sroufe and colleagues3 report on persons from adverse socioeconomic circumstances who were studied from birth into young adulthood. The investigators describe the development and use of age-appropriate techniques to get at the nature of the individual's internal representations. What is very exciting about their findings is that these measures of representations substantially predict behavior at the next developmental stage, with those experiences at that developmental stage predicting the nature of the internal representations at the following developmental stage. Thus, the inability of Reiss's study to identify internal representations as part of non-shared environmental influences is addressed in the longitudinal study by Sroufe and colleagues.
All of this is (or should be) most exciting to clinicians of diverse theoretical orientations. These 2 remarkable research projects have begun to document the central importance of the ways in which relationships are experienced, stored, recalled, and narrated. Whether we use the language of internal representations, internal working models, cognitive schemata, or core assumptions and expectations, a fundamental clinical premise derived from various psychotherapies is being documented by systematic longitudinal research.
How sweet it is.