I think it can be proved its why women and men undertake physically and mentally grueling attempts at fertility treatment as described in KM's book. Its why same sex couples such as Elton John and David Furnish turn to surrogacy. Its why the likes of my adoptive parents adopted me.
I believe the real reason for Elton and David's surrogacy was neither of them had a w..b. kinda need one of em for babies.
Instinct. I do believe in humans having a 6th sence. we lose it if we don't tune into it. I know when any of my children are having a difficult day or are upset before they come home from school! thoughts about my child would be in my head and I would get a sad feeling for no reason. And they all have a cheerie disposition so it isn't an every day occurance.
I do believe in mothers instinct- not all mothers are aware of it. this can be due to conception/difficult pregnancy/pre/postnatal depression to name a few. Some mothers hate their offsprings or have favorites.