> >would want to hold an infant on the left to leave the right hand free to
> >rock, spear, or other weapon. I would venture to guess that the vast
> >of people and apes will hold a baby on the non-dominant side to leave
> >dominant hand free, whether the individual is right or left handed.
> If that were the case why would lefthanders show exactly the same bias??
This hypothesis originates with Dr. Bill Calvin who suggests that the reason a mother would want to hold a baby on the left side is because that is where the heart is most audible. Babies held on the left side tend to be much quieter due to the heart sounds and he supposes that this would make a right handed mother a more succesful hunter because the right hand would be free when the baby is quiet. He admits this is pure speculation though and that the essay was initially written as a parody of the right handed male warrior hypothesis (the shield is carried on the left side to protect the heart) that is almost certainly untrue. See his book "The Throwing Madonna" http://weber.u.washington.edu/~wcalvin/bk2/bk2.htm for further referrence.