I dati che citi sono corretti, a quanto mi risulta. Tuttavia, la questione è - a mio parere - più complessa di come la poni tu.
Ipotizzare peraltro un legame causale tra vantaggio genetico differenziale e comportamento umano è un'impresa veramente titanica

Da un punto di vista storico, la pratica del fosterage sembra andare in direzione contraria alle tue tesi.

Like other children who grew up in polygynous families, Sylvie distinguishes her biological mother’s behaviour from that of her mothers’ co-wife. She calls her mother’s co-wife “stepmother” which she explains as “bad mother”. She even accuses her of sorcery

Sylvie further explains that the stepmother, who always gave preference to her own children, gave cause to arguments between the children in the compound. Sylvie explains that there is no sense of confidence or solidarity between half-brothers and half-sisters and that she can only trust her brothers and sisters who are born from the same womb. Sylvie’s mother tells me that she took her daughter away from the stepmother and fostered her out to other parents in order to put her child away from this hostile atmosphere. The mother herself stayed behind with her husband and co-wife. Another story, that of Viviane, show us that mothers who leave the compound of their husband, also want to protect their children from the co-wife.

A local saying is that children can not develop in the hands of their stepmother. Generally it is said that stepmothers delay their progress at school by using them as slaves who can be sent on an errand every moment a day or can be forced to do small jobs. The stepmother hinders progress at school because she does not want them to find a good job to be able to maintain their mother when she is old. Children also complain that the stepmother, especially when their own mother is not at home, does not feed them. Another general saying is that the stepmother does not pay medicine when a child of her co-wife falls ill, neither takes it to hospital in case of need. Moreover, both mothers and children fear that the stepmother will use sorcery to poison the child of her co-wife.

In this context of polygyny fosterage can not entirely be explained on the basis of economic reasons. Neither can it simply be explained as a mother’s shortcoming or as an attitude of indifference towards her children. It can rather be seen as an expression of a mother’s feeling of solidarity with her children, and especially her daughters. First wives in polygynous marriages frequently use fosterage to separate their children from the co-wife and to protect them from her. In this way, fosterage can be seen as a strategy to be competent mothers.

In Dutch society, the only reason why children have to leave their parents and to join other educators, is that the biological parents are incapable of educating their children themselves, for example, since they have psychological problems or are addicted to drugs. The perception of the biological mother as the only good mother is so dominant that both the mother as her children are supposed to suffer deeply when separated: Women would suffer from being seen as inadequate mothers, and children would suffer from lack of maternal care and love.
Recent studies that confirm the normative view that children are better of as long as they stay with their biological parents are Wegar (1997), and South, Crowder, and Trent (1998). Wegar (1997) explores representations of adoptive mothers in the United States from the 1940s to the 1990s and states that adoptive mothers have been depicted as deviants who fail to conform to dominant definitions of true womanhood and good mothering. South et all (1998) state that parental divorce sharply increases the probability that children will move out of their neighbourhood and that this mobility decreases the well-being of children. Both studies consider biological parenthood and the stable, nuclear family as necessary for children’s best educational environment.