I attempted earlier to respond to MBC's interesting observations, but must have pushed the wrong button! Bad genes, perhaps.
The observations on separated sibs are extremely important. Certainly similarities in behavior can reflect similarities in genetic foundations.
Two quick points:
1) Non identical sibs share "50%" of their genes, in evolutionary genetics jargon. Thus, sibs - even reared under identical conditions (a theoretical comment; impossible in practice) - can vary greatly as a reflection of these genetic differences. Interestingly, in this case the DIFFERENCES rather than similarities would point to genes.
2) Sibs, even when subsequently adopted, might have shared critical experiences in their early development. These experiences can occur from birth, and even before (e.g. mother's physiology). Thus similarities seen after the sibs are in different environments might also reflect common early experience. In this case SIMILARITIES rather than differences might point to experience.
However, neither of these comments negates the value of close observations of sibs reared apart. I would be very interested in contributions of others to this fascinating area.
The cleanest examples come from identical twins, but it is important to remember that even in these cases shared early (pre-adoption) experiences can make the interpretation less clear than one might expect. Animal studies can help us move away from some of these difficulties, but in this case extrapolations across species must be made with caution. This is especially true for our "higher-order" human attributes.
Sorry I goofed on my previous attempt to reply, and thus the truncated nature of the present note.