Abstract
Temperamental variation in infancy was the starting point for this longitudinal study of twins followed frombirth to adulthood. The influence of early individuality on later development was a research question in thefollow-ups, and by choosing twins as a group, it was possible to uncover the effect of environmental andgenetic factors at the different ages. Developmental psychopathology turned out to be a relevant framework,where mental health and well-being are seen as the result of a continuous transaction between geneticallybasedindividuality and a changing and varying environment (Maughan & Rutter, 2008). The twin methodwas an adequate tool to gain a better understanding of these processes. The overall clear message from thestudy is that whereas heritability of temperament can be seen from infancy on, environmental factors are ofcrucial importance for an understanding of mental development and health, but the effect of environmentalfactors probably varies a great deal in dependency upon the genetic disposition of the person.
Highlights
Contrary to most longitudinal twin studies described in today’s literature, where a large number of twins are seen by a large number of professionals, this longitudinal twin study examines only one small group of 53 pairs of twins born in 1969 and 1970 and seen mainly by one person
Earlier well known longitudinal studies on child development, as conducted by Nancy Bayley, or Arnold Gesell, both started in the 1930s, typically focused on normative development: What is the normal stage of development of a child at each age? The New York Longitudinal Study (NYLS) started by Thomas and Chess and their colleagues in 1956 (Thomas et al, 1963) focused on variation in behavior from infancy on as an input to the transactional processes with the environment throughout development
Thomas and Chess focused on temperamental individuality from infancy, they did not say anything about the reasons for the variations
Summary
Contrary to most longitudinal twin studies described in today’s literature, where a large number of twins are seen by a large number of professionals, this longitudinal twin study examines only one small group of 53 pairs of twins born in 1969 and 1970 and seen mainly by one person (the person who conducted this study and authored this contribution).Within developmental psychology, as within social sciences in general (Kuhn, 1962), there was a paradigm shift in the late 1960s from linear to transactional explanation models of development. The New York Longitudinal Study (NYLS) started by Thomas and Chess and their colleagues in 1956 (Thomas et al, 1963) focused on variation in behavior from infancy on as an input to the transactional processes with the environment throughout development. Thomas and Chess underlined that already in infancy, the norm was to be different in behavior, which they initially called “primary reaction pattern” or “initial reactivity”. They eventually identified nine different behavior styles along which infants were normally distributed, which they later called “temperament”, and by this pioneered the systematic applications of temperament concepts to measurement of children’s individuality.
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