We live in a world of Yin and Yang. Where good will never overtake evil and evil will never overtake good. The flaw of this world is that 50% evil is too much and 50% good is too little.
The authors are saying, in essence, that family environments exert less influence over who the kids grow up to be than do the genes they inherit from birth. Understandably, most parents do not want to hear or believe this.