But here I stop—short of any deterministic speculation that attributes specific behaviors to the possession of specific altruist or opportunist genes. Our genetic makeup permits a wide range of behaviors—from Ebenezer Scrooge before to Ebenezer Scrooge after. I do not believe that the miser hoards through opportunist genes or that the philanthropist gives because nature endowed him with more than the normal complement of altruist genes. Upbringing, culture, class, status, and all the intangibles that we call free will, determine how we restrict our behaviors from the wide spectrum—extreme altruism to extreme selfishness—that our genes permit.

"So Cleverly Kind an Animal", p. 266 - Ever Since Darwin (1977)