Findings of behavioural genetic studies show, that environmental factors that have the strongest effect on child development are those which make siblings in the same family different from one another, i.e. environmental influence of primary importance is not shared by members of the same family. Child specific experiences within the family include also parental differential treatment. This concept describes the behaviour of parents towards one of their children as compared to their behaviour towards child's sibling. This parental behaviour is related to certain characteristics of their children's development, e.g. their social adjustment and sibling relationship, though these relations are predominantly low and indirect. Parental differential treatment is not necessarily a negative process in the family, as it can reflect parental sensitivity to individual differences between their children. Also, the effects of parental differential treatment on child development are most probably negative only if it is extremely pronounced and consistent across members of the family system. The size of parental differential treatment as well as its correlates depend also on characteristics of both children (e.g. gender, personality characteristics) and family context. In addition, at least in middle childhood and adolescence, child/adolescent perceptions of parental differential treatmnet as fair or unfair seem to play more important role in child/adolescent development than the size of such parental behaviour.