The family's influence on individual differences in context is part of a larger question, an old question – older than developmental science: What is the role of experience in development? Consideration of the importance of any influence on development inevitably leads to considerations of nature and nurture, because these are the only two mechanisms that can explain individual differences in development. From the beginning, theorists have questioned whether the effects of experience can be isolated. In 1934, Gesell and Thompson concluded that nature and nurture could be separated “only in analytical thinking.” More recently, Gottlieb, Wahlsten, and Lickliter (1998) argued that attempts to separate the two were “nonsensical.” Yet, polarizing claims continue. One example is Harris's (1995) thesis that families do not matter. This conclusion was based primarily on the failure to find strong shared environmental effects for children's psychological development. Critiques of Harris's thesis have focused on the likelihood that family influences are largely not shared, because parents respond to individual differences among their children (Rutter, 2002; Vandell, 2000). The most promising research today on the family's influence concerns genotype-environment interplay, rather than research that promotes one over the other.

I have recently argued that theory and conceptualization in developmental science far exceed methodology (McCartney, 2003). This is hardly controversial. Behavior genetics and socialization methods are each limited, for reasons that are well known to developmentalists (Collins, Maccoby, Steinberg, Hetherington et al., 2000).