"However, further research has shown that it is the normal condition for humans and for most other mammals. It seems pretty clear why this is the case for most mammals and for most human beings"

Marvin Harris is recommending that monogamy is the regular condition for human beings and most other mammals. This is likely due to the reality that monogamy provides a steady environment for raising offspring. It is useful for both parents to be involved in the childhood of their children, as it increases the opportunities of survival for the young. Monogamy likewise provides a sense of security and stability for both partners, as they are both dedicated to the relationship. This is helpful for both partners, as it enables them to focus on other elements of their lives, such as career and personal development. Monogamy is likewise helpful for the types as a whole, as it helps to ensure the survival of the types.