INTRODUCTION

Perhaps surprisingly, children can be exposed to very similar life experiences and yet they will be affected by these experiences in very different ways. In other words, similar environmental experiences do not result in children developing more similarly to one another. Several types of evidence suggest this. One type of evidence comes from twin studies in which it is possible to partition variance into genetic and environmental influence. Such studies show that once genetic effects have been controlled, siblings tend to be more dissimilar than similar on emotions and behavior (Plomin & Daniels, 1987). This is the case even though siblings are raised in the same home and exposed to, we assume, many of the same environmental influences. This suggests enormous variability in the ways in which individuals respond to environmental influences. Surprisingly, this is also the case at high levels of psychosocial adversity. We might think that being raised in a highly stressful environment would have an adverse effect on all children. It is clear, however, that this is not the case (Luthar, Cicchetti, & Becker, 2000). Even under highly adverse conditions such as living through wars in which loved ones are killed (Howard & Hodes, 2000) or being raised by parents with serious mental health problems (Jaffee et al., 2003; Niemi et al., 2004) there is still variability in children's responses to such stressors. For some, the exposure is associated with compromised development. For others, no evidence for behavioral or emotional compromise is evident.