One observation that makes the findings under consideration particularly interesting is that short alleles on the serotonin transporter gene have also been found, in at least some molecular-genetic research,33 to be associated with negative emotionality in young infants. What makes this gene–behavior linkage particularly important is that, even though negative emotionality or difficult temperament in infants has long been conceptualized as a risk or vulnerability factor for the development of behavior problems in childhood,34 a growing literature on differential susceptibility reveals negatively emotional or difficult infants to be more plastic or malleable than other infants—in a for-better-and-for-worse manner.35 That is, they do worse than others under poor rearing conditions, but better than others under good ones. Consider in this regard the evidence that infants rated by mothers as highly negatively emotional at 6 months of age not only manifest, relative to other children, more behavior problems in early childhood when experiencing low-quality parenting36 or low-quality child care,37, 38 but also fewer problems and more social skills than other children when exposed to high-quality parenting or child care. Relatedly, Kochanska et
