Human beings have evolved similar physical attributes over time. Though there is some genetic variation among individuals, we share many traits. But isn't it also possible that, as a result of our common evolutionary heritage, we share similar emotional and moral traits as well? If we all have basically similar emotional machinery, why couldn't we appeal to the general constellation of desires that most of us share, and use them to construct a universal ethics? If the good is what makes us happy, and happiness is the fulfillment of various desires, and if humans have similar desires because we share evolved mental traits, then why couldn't an appeal to those traits in the search for moral agreement? Just as medical experts can give general advice about physical health because most humans share similar physical bodies, why can't psychologists and ethicists give general advice about morality based upon our shared mental traits?

We do have a lot in common psychologically, and all of that matters when we're trying to decide what's right and wrong. And the more we know about the psychological effects of how we treat people, the more information we'll have to feed into our ethical decisions. Psychologists have relevant things to say, as do doctors and, for that matter, economists, massage therapists, and various other specialists.

Whether or not knowing everything about what makes people happy would settle all ethical questions, however, is another matter. (Not sure if you were suggesting it would.) For example: suppose that there are things that would make me happy at your expense. Most of us don't think it's just a matter of comparing the sum of my potential pleasure to the sum of your potential pain. Questions about fairness, for example, will also matter, and psychologists have no special expertise in sorting out what's fair. (Neither do most philosophers, for that matter.)

There's also room to argue about whether the good really is just what makes us happy, and about whether happiness is a just a matter of satisfying various desires. And in fact, we've only scratched the surface. So while the sorts of things you point to are bound to matter for ethical matters, there will be plenty left over that knowing how people's psyches work won't tell us.

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