"Feral children" is certainly an interesting counter-example to show the effect of society on human development, but it is fraught with a problem - it demonstrates the effects of the absence of parental care and through them of society, rather than society in general. We need to analytically separate the effect of parental care from the effect of social organization which is being "transmitted" via parental care. Such hypothetical counter-exapmpe would involve children of "feral children" i.e. children brought up by parents who had no contact with a human society. It is, however, almost impossible to find such an counterexample.
Another problem with "feral children" as a counter-example demonstrating the effect os human society is that cognitive capacity of a feral child cannot be observed directly - it is usually assessed by observing perfomance of some culturally defined tasks, and that is a very biased assessment (see Werner Herzog's film "Jeder fuer Sich und Gott Gegen alles" telling the true stroy of a "feral child" Kaspar Hauser). In other words, it is possible that human mind under "feral condition" developes different cognitive capacity from that under "civilized" condition - and the two cannot be compared as "better" or "less" developed. The problem is akin to that found in anthropology studying "primitive" cultures that develop cognitive capacities that are much different than but not necessarilii inferior to those found in "civilised societies" (cf. Claude Levi-Strauss, _The Savage MInd_ and another Werner Herzog's film "Where the Green Ants Go" (I'm not sure if I remember the title correctly).
The bottom line is that while there is no true counterfactual to demonstrate the effect of the absence of society on human development, we can take a different approach, known as the Mill's method of concomittant variance and study the effect of different social structures on human cognitive development. That approach was suggested by Emile Durkheim in _The Elementary Forms of the Religious Life_.
wojtek