Thanks for this link. That helped me understand why group selection may be intuitive but is most likely wrong. To summarize, group selection can work mathematically, but you must carefully choose the parameters of the mathematical model in order for "grep-selective" behavior (i.e. altruism toward strangers) to be selected. (These parameters define things like the cost and benefits of altruism to the "giver" and "receiver".) The slightest variation in these parameters results in a collapse of altruism and reversion to selfishness. So while it is theoretically possible, in practice the incentives just don't line up in a way that selects group-selective behaviors over selfish ones.