Marg0,
Have you read what Dawkins has to say about 'MEMES'.
Lower animals pass on the bulk of their investment through 'genes' and so are naturally agressive towards unrelated offspring of a partner. In human society, memes play a greater role and so step parents can still pass something on to unrelated offspring if they have an influence on their upbringing. That is more likely to be the case if the children are young when the step-relationship begins.
Another factor that would geneticaly select for a caring step parent is that if the step-parent can prove themselves to be a caring provider, then the other parent is more likely to go on to produce more offspring in the relationship. Agressive behaviour towards step-children is more likely to mean the relationship is going nowhere.
The difference between the majority of animals and humans is that human beings have the most complex long-lasting social structure based mostly on long-term relationships.
Finally, figure, What is the basis of your statement Quote: "I should say there are more abusive step-parents than there are kind and caring step-parents."?
My own experience of step-parents is that most are caring and non-abusive. I don't understand why this reply got 3 stars?