What, precisely, constitutes "ethical" behavior?
Update: Based on comments, I see I've omitted a important qualifier. I'm talking here only about what constitutes the general domain of behavior that pertains specifically to ethics, not about what constitutes good ethical behavior or behavior I would always approve of.
Kant is notorious for defining (or appearing to define) a behavior as "ethical" if and only if the behavior is not motivated in any way, directly or indirectly, by one's own self-interest. Divine Command Theory holds that ethical behavior is compliance with the commands of God. Obviously, by the latter definition, an atheist cannot act ethically; she can at best only simulate ethical behavior; the requisite motivation is necessarily absent. The definition attributed to Kant seems, on basic reflection, physically impossible given the goal-seeking nature of human minds. Although there are religious atheists (e.g. Buddhists), an atheist committed to naturalistic physical monism (a.k.a. scientific materialism) would thus usually reject Kant's definition.
Divine Command Theory is not really subject to rational discussion. No one has improved on Socrates' Euthyphro paradox, and Leibniz' comment,
In saying, therefore, that things are not good according to any standard of goodness, but simply by the will of God, it seems to me that one destroys, without realizing it, all the love of God and all his glory; for why praise him for what he has done, if he would be equally praiseworthy in doing the contrary?still has considerable rhetorical force.
In any event, adherence to Divine Command Theory simply excludes a priori atheists and members of other religions (for following the wrong commands, not God's) as ethical agents and establishes a fundamental irreconcilable enmity.
Any naturalistic account of ethical behavior must be predicated on observation: What sorts of behavior do we typically call "ethical"? Alternatively, we can construct arbitrary definitions of "ethical" that allow us to create falsifiable theories that account for human behavior.
One definition of "ethical" that seems scientifically fruitful as well as—at least to me—blindingly obvious:
A behavior is "ethical" if and only if it has the foreseeable effect of promoting others' self-interest at the expense of some specific self-interest of the agent.This definition is a much weakened version of Kant's too-strong definition. Instead of being performed entirely without regard to self-interest, a behavior must merely be contrary to only some self-interest (as well as promoting others' self-interest) to be definitionally ethical.
This definition draws a distinction. It distinguishes behavior which does not promote the self-interest of others, and it distinguishes behavior which, although promoting the self-interest of others, is not contrary to any of one's own self interest. For instance, seeing a movie with a friend would not qualify as a specifically ethical act, since even the value of the companionship itself to one's friend is not (assuming one also values companionship) achieved at any cost at all to one's own self-interest.