Harris is dead wrong at every level, and he is blind to his own cultural biases. First, in an atheist cosmos (which Harris has previously advocated), there can be no inherent morality because the universe does not care. The cosmos takes no account of humankind and cares not a blip whether the human species lives or dies. Whether we lie, cheat, steal, or kill makes no difference to the stars and black holes, and therefore have no inherent moral value except what we--through our cultures--assign them. Stripped to is elements, therefore, Harris's argument is merely that science can tell us whether certain policies will lead to outcomes Harris views as favorable, such as increased lifespans, healthier bodies, etc. But this is not a universal moral imperative; it is the preference of a certain type of upper middle class, twenty-first century American who seeks to universalize his own preferences under the guise of scientific objectivity.
But let us grant Harris his argument. Under his reasoning, therefore, there is a strong scientific case that systematic human population reduction, through freeing up resources, will make humanity happier and healthier. Therefore, policies of mandatory sterilization are entirely justified. Where shall we start with the castrations? I vote to start with Harris, but will he agree? After all, science says to do it. Right?