During the last decade, a revolution has taken place among clinical and developmental psychologists. Whereas the emphasis was formerly on continuity and stability over time, with earlier experiences uniformly shaping developmental trajectories, many researchers have come to recognise that individuals are not uniformly plastic, so that some are more susceptible than others to influence. In perhaps the most widely cited demonstration of this, Caspi and colleagues showed in a retrospective study in New Zealand that individual differences in a specific genetic allele determined which children would be adversely affected by depriving early life experiences, whereas children without this allele emerged apparently unscathed from similarly debilitating experiences. The clarity of this finding quickly made believers of many researchers who had hitherto viewed with scepticism decades of scholarship on risk and vulnerability by such researchers as Rutter, Gottesman, and Masten, but empirical demonstration of the nature, extent, and limits of susceptibility to influence (both vulnerability and its obverse, strength in the face of adversity) has been limited. The goal of the proposed research is to examine multiple possible sources (both genetic and environmental) of these individual differences and an array of developmental outcomes using a large, representative, and rich data set collected by the ALSPAC team.

For many years (and with differing degrees of emphasis at different times on either side of the Atlantic) students of child development have recognised that psychological and behavioural development are neither predetermined nor infinitely malleable, although infants appear to be born with marked congenital individual differences. Child psychiatrists such as Thomas and Chess devoted their careers to illustrating the impact of such inborn temperamental attributes on children's developmental trajectories, whereas psychologists such as Rothbart have sought to understood the physiological bases of the major dimensions of temperament that have been identified (Rothbart & Bates, 2006). Students of temperament and behaviour genetics have thus emphasized the importance of biogenetic predispositions and have sought to unpack the ways in which experience might, in combination with congenital individual differences, shape children's development, by promoting stability over time (i.e., 'the child is father to the man'), leading some children to develop more poorly than anticipated, or ensuring that some children prosper, doing better than originally expected. To some extent, progress has been impeded by measurement difficulties, particularly by the challenge of making sense of differences between temperamental appraisals provided by parents (reflecting their subjective perceptions of the child) and those obtained through more objective and systematic observational and physiological means.

In addition, the focus has largely been on the temperamental factors that place some children at greater risk of developing behaviour problems, including serious psychological disturbances-especially because they are "difficult" or "irritable"-- and upon the stability of individual differences, that is, the degree to which children remain difficult across infancy, childhood and adolescence. By contrast, considerably less attention has been paid to such positive characteristics as cheerfulness, perseverance, the capacity to love and be loved, or resilience that might presage positive outcomes; and even less empirical work has focused upon the extent to which-and the reasons why-children's characteristics change. Even though developmental psychologists have shown more interest in the positive side of life than have clinical psychologists, investigating such topics as the origins of empathy, prosocial behaviour, and social skills, few researchers have explored why some children change from negative to positive developmental trajectories and vice versa.

Although it is seductive to assume that those most positively or negatively disposed early in life remain that way as they age, new findings reported by Sharp, Croudace, and Goodyer (2007) caution against such potentially simplistic reasoning. Not only were optimistic 7- to 11-year-olds more likely to change their orientations than peers who had an initial 'neutral' orientation, but those who initially scored highest on optimism scored highest on psychopathology two years later. Clearly, one cannot presume that continuity always characterises the process of human development, including positive development.

Attachment theory has played a particularly important role in shaping understanding of the ways in which early experiences (notably, the quality of parental behaviour and of infant-parent interaction) influence the quality of the close, emotional relationships that infants form to each of their attachment figures and which, in turn, help shape children's later social relationships and approaches to other challenging tasks and experiences. Over the last 40 years, and especially since Ainsworth developed the Strange Situation Procedure (SSP) to assess the security of infants' emotional ties ('attachments') to their mothers, fathers and other caregivers, developmental psychologists have made considerable progress in illuminating the origins and predictive implications of individual differences in child development, even those measured very early in life.