Evolutionary psychology has made impressive progress in understanding the origins of morality. Along with the many posts about these origins on Less Wrong I recommend Robert Wright's The Moral Animal for an excellent introduction to the subject.

Guilt does not naturally fall out of these explanations. One can imagine a mind design that although often behaving morally for the same reasons we do, sometimes decides a selfish approach is best and pursues that approach without compunction. In fact, this design would have advantages; it would remove a potentially crippling psychological burden, prevent loss of status from admission of wrongdoing, and allow more rational calculation of when moral actions are or are not advantageous. So why guilt?

In one of the few existing writings I could find on the subject, Tooby and Cosmides theorize that "guilt functions as an emotion mode specialized for recalibration of regulatory variables that control trade-offs in welfare between self and other."

If I understand their meaning, they are saying that when an action results in a bad outcome, guilt is a byproduct of updating your mental processes so that it doesn't happen again. In their example, if you don't share food with your sister, and your sister starves and becomes sick, your brain gives you a strong burst of negative emotion around the event so that you reconsider your decision not to share. It is generally a bad idea to disagree with Tooby and Cosmides, but this explanation doesn't satisfy me for several reasons.

First, guilt is just as associated with good outcomes as bad outcomes. If I kill my brother so I can inherit the throne, then even if everything goes according to plan and I become king, I may still feel guilt. But why should I recalibrate here? My original assumptions - that fratricide would be easy and useful - were entirely correct. But I am still likely to feel bad about it. In fact, some criminals report feeling "relieved" when caught, as if a negative outcome decreased their feelings of guilt instead of exacerbating them.

Second, guilt is not only an emotion, but an entire complex of behaviors. Our modern word self-flagellation comes from the old practice of literally whipping one's self out of feelings of guilt or unworthiness. We may not literally self-flagellate anymore, but when I feel guilty I am less likely to do activities I enjoy and more likely to deliberately make myself miserable.

Third, although guilt can be very private it has an undeniable social aspect. People have messaged me at 3 AM in the morning just to tell me how guilty they feel about something they did to someone I've never met; this sort of outpouring of emotion can even be therapeutic. The aforementioned self-flagellators would parade around town in their sackcloth and ashes, just in case anyone didn't know how guilty they felt. And we expect guilt in certain situations: a criminal who feels guilty about what ey has done may get a shorter sentence.

Fourth, guilt sometimes occurs even when a person has done nothing wrong. People who through no fault of their own are associated with disasters can nevertheless report "survivor's guilt" and feel like events were partly their fault. If this is a tool for recalibrating choices, it is a very bad one. This is not a knockdown argument - a lot of mental adaptations are very bad at what they do - but it should at least raise suspicion that there is another part to the puzzle besides recalibration.

THE PARABLE OF THE LAWYER

Suppose you need a lawyer for some important and very lucrative legal case. And suppose by a freak legislative oversight, your state has no laws against legal malpractice and unethical lawyers can get off scot-free. You are going to want to invest a lot of effort into evaluating the morals of the many lawyers anxious to take your case.

One lawyer you meet, Mr. Dewey, has an unusual appearance. A small angel, about the size of a rat, sits on his right shoulder holding an electric cattle prod. This is remarkable, and so you remark upon it.

Mr. Dewey scowls. "That angel has been sitting there for as long as I can remember," he tells you. "Every time I do something wrong, she pokes me with her prod. If it's a minor sin like profanity, maybe she'll only poke me once or twice, but if I lie or swindle, she'll turn the power up on max and keep shocking me for days. It's a miserable, miserable existence, and I'm constantly scared to death I'll slip up and make her angry, but I can't figure out how to get rid of her."

You express some skepticism about this story, so Mr. Dewey offers to demonstrate. He says a mild curse word, and sure enough, the angel pokes him with the cattle prod, giving him a mild electric shock.

Suddenly, Mr. Dewey is a very attractive candidate for your lucrative case. You can be assured that he won't swindle you, because whatever gains he might take from the swindle are less attractive than the punishment he would get from the angel afterwards.

Surgeon Paul Brand considered pain so useful to the body's functioning that he called it "the gift nobody wants". Mr. Dewey's angel is also such a gift, even though he might not appreciate it: clients worried about ethical issues will bring their patronage to his law firm, giving him a major advantage over the competition.

Whereas normally we must trust a lawyer's altruism if we expect em not to con us, in Mr. Dewey's case we need only trust him to pursue his own self-interest. This, then, is the role of guilt: it provides assurance to others that we will be punished for our misdeeds even if there is no external authority to punish us, avoiding Parfitian hitchhiker  dilemmas and ensuring fair play. The assurance of punishment ensures fair play and makes mutually beneficial transactions possible.

FAKEABLE AND UNFAKEABLE SIGNALS

The big difference between Mr. Dewey and ourselves is that where Mr. Dewey has unquestionable evidence of his commitment to self punishment in the form of a very visible angel on his shoulder, for the rest of us guilt is a private mental affair and can be faked. It would seem to be a winning strategy, then, to claim a tendency to guilt while not really having one.

Ms. Wolfram is Mr. Dewey's main competitor, and is outraged at her rival's business success. In an attempt to even the scales, she buys a plastic angel figure from the local church and glues it to her shoulder. "Look!" she tells clients. "I, too, suffer pain when I commit misdeeds!" Her business shoots up to the same high levels as Mr. Dewey's.

One day, the news comes that Mr. Dewey was spotted whipping himself in the town square. When asked why, he explained that in a moment of weakness, he had overcharged a customer. His angel, who had lost its cattle prod, was mind-controlling him into the self-flagellation in place of its more usual punishment.

This provides an impressive bar for Ms. Wolfram to live up to. Sure, she could just whip herself like Mr. Dewey is doing. But it wouldn't be worth it - she just doesn't like the money enough that she would whip herself after every swindle just to drum up business. If she's going to have to whip herself to fake remorse whenever she commits wrongdoing,  her best policy really is to genuinely stop swindling people.

Mr. Dewey has found an unfakeable signal. Even though whipping himself in public is one of the most unpleasant things he could do, in this case it is good business practice. It once again differentiates him from Ms. Wolfram and restores his status as the city's most desirable attorney.

In evolutionary terms, guilt becomes more credible the more it requires publicly visible behavior that no reasonable cheat would want to fake. Hurting oneself, avoiding pleasurable activities, lowering your own status, and withdrawing from social activities are all evolutionary costly and therefore good ways to prove you are experiencing guilt; the usual vocal, postural, and facial cues of being miserable are also useful.

There's no reason people should evolve an all-consuming sense of guilt. If an opportunity comes along where the benefits of cheating are greater than the social costs, an organism should still take it. Therefore, guilt has to be unpleasant but not infinitely unpleasant. A person who committed suicide in response to even the slightest moral infraction would be trustworthy, but they'd miss out if an excellent opportunity to win major gains for cheating happened to fall into their lap.

The conspicuous experience of guilt is an evolutionarily advantageous way of assuring potential trading partners that you will be punished for defection. The behaviors associated with guilt are costly signals that help differentiate false claims of guilt from the real thing and add to public verifiability of the punishment involved.

UNDESERVED GUILT

If you kill your brother in order to inherit the throne, you probably deserve whatever guilt you feel. But in the phenomenon of "survivor's guilt", people feel guilt for events that weren't even remotely their fault. Maybe you go hiking with your brother, and through no fault of your own he trips and falls down a crevasse and dies, and now you feel guilty. Why?

Hunter-gatherer societies were more violent than our own; statistics differ but by some estimates around 30% of hunter-gatherer males died of homicide. Even as late as the Bronze Age, Biblical figures who killed their brothers comprise a rather impressive list including Cain, Solomon, Ammon, Abimelech, and Jehoram; Jacob's sons merely attempted to do so. So the priors for suspicious death must have been very different in the olden days.

Further, in such a crime-ridden culture, there may have been more incentives to blame an enemy for a death, even if that enemy was not responsible. A person whose brother has accidentally died on a hiking trip with no witnesses would be very targetable.

And even in less drastic situations than blaming survivors for a death, there may be other possible threats to reputation. If there is only one survivor of a battle, he may be suspected of cowardice; if there is only one survivor of a disaster, she may be suspected of running away without helping others.

Therefore, it would be advantageous to have a method of proving your innocence. Suppose that you would gain benefits X from killing your brother and covering it up, but that you would suffer losses Y if you were suspected of the crime and punished. A precommitment to a policy of experiencing a level of guilt between X and Y provides a tool for proving your innocence. It would no longer be in your self-interest to kill your brother, because you will suffer so much guilt that you won't be able to enjoy the benefits of your crime; your would-be accusers realize this and admit your innocence, saving you from the still worse outcome Y.

In this case, guilt would be an entirely adaptive response to a disaster with which you were associated, even if your own actions were beyond reproach. A level of unhappiness worse than any benefits you could get by profiting the tragedy, but less than any punishment you might receive if you were suspected of profiting from the tragedy, would be helpful in clearing your name of any wrongdoing.

(The proposed mechanism is almost identical to one cited in Thornhill and Palmer's controversial and unpleasant evolutionary account of post-traumatic stress after rape.)

This theory makes some testable predictions, which as far as I know have not been tested:

- People should feel guiltier about events for which reasonable suspicion might exist that they played a part; for example, if your brother slipped and fell while you were hiking alone with him rather than in a large group with many witnesses.
- People should feel guiltier about events for which they might profit; for example, if you stood to inherit money from your brother, or never liked him much anyway.
- People may be suspicious of people who come out of a disaster feeling no survivor's guilt.

CONCLUSION

Guilt, like pain, is "a gift nobody wants". Because people with guilt are known to punish themselves for moral wrongdoing, their social group considers them more trustworthy and they gain the advantages of trade and cooperation. In order to prove that their guilt is real rather than feigned, they use costly signals like deliberate self-harm and self-denial to display their punishment publicly

When one has done nothing wrong, it can sometimes be advantageous to paradoxically display guilt in order to prove one's lack of wrongdoing. These costly signals demonstrate that it is not in one's self-interest to lie about these matters, while still being less costly than the punishment for defection.

Although this could theoretically be mediated by the behavioral strategies of a sufficiently intelligent and Machiavellian unconscious mind, it fits within the framework of evolutionary psychology and can also be interpreted in evolutionary terms.