This is an essay about individual rationality technique. It's intended to show that "All interesting behavior is overdetermined" is, under a specific meaning of "overdetermined" that is stronger than just "multicausal", a valuable heuristic for generative understanding and the avoidance of error.

Scott Alexander's 2018 essay The Omnigenic Model As Metaphor for Life is a clear statement of what has become general rationalist doctrine on multicausality. Scott observed that behavioral genetics was difficult because complex human traits involved far more interacting causes than workers expected. He draws this methodological lesson: “We should go from trying to discover ‘the’ cause, to trying to find out which factors we need to create the best polycausal model.” He applies the same reasoning to depression, intelligence, comma, and other complex psychological phenomena. He explicitly warns against looking for one or ten causes when there might be dozens or hundreds.

Scott is, however, talking mainly about contributory causation, when an effect is produced by an accumulation of causes; in his type example, there are probably twenty thousand genes which contribute to intelligence, but no one "intelligence gene". This is not quite the same as overdetermination, which describes a situation in which a complex behavior could be explained with any one of multiple causes, each one in itself sufficient.

When we find a sufficient explanation, or a sufficiently good collection of contributory explanations like Scott's 20,000 genes, there's a strong tendency to think that we can stop there. The rest of this essay is about why we shouldn't.

I first came to understand the importance of overdetermination when I was studying primate ethology. There is a 1987 paper by Hausfater and Takacs, Structure and Function of Hindquarter Presentations in Yellow Baboons (Papia cynocephalus). It examines a striking feature of baboon social behavior: individuals turn their hindquarters toward another animal, sometimes assuming a posture resembling female sexual solicitation, even when the interaction has nothing to do with mating.

The authors analyzed 775 presentation sequences recorded during 603 hours of observation of two groups of wild yellow baboons in Amboseli, Kenya. Taking into account the histories of the individuals involved, they classified these interactions into a number of categories: submissive, sexual, mixed sexual-submissive, affiliiative, and grooming.

The most frequent category of interaction was submissive (intended to avert aggression) at about half. About one-third were sexual or mixed sexual-submissive. The authors proposed that hindquarter presentation originated principally as a sexual solicitation signal, but subsequently acquired an important secondary function: dissipating aggression by a dominant individual.

Hindquarter presentation is overdetermined. For each individual presentation, one of several motivations could be a sufficient causal explanation. In some cases, only a combination of multiple causes ("sexual and submissive") is sufficient explanation. If you were to stop with any one sufficient explanation of the phenomenon as a whole, you couldn't ever come anywhere close to a causal theory of why hindquarter presentation does different things at different times.

Hausfater and Takacs did not use the term "overdetermination". However, it has shown up in similar contexts. In Primate Parenting (1990) we see this: “Maternal behavior is obviously so important to the survival of a species that it has been ‘overdetermined’—that is, driven by multiple behavioral and physiological systems.”

This amplifies Hausfater and Takacs's proposal that the meaning of hindquarter presentation has broadened and shifted under evolutionary selection. It hints at a general principle that took me years to properly formulate:

In adaptive systems evolving under selection and scarcity, behaviors that serve multiple purposes are efficient. Therefore, they are favored. A behavior, once developed, will tend to accumulate uses if it can — that is, when the marginal cost of adding an adaptive function to an existing behavior is less than the cost of evolving and maintaining a new and separate behavior. It will then become overdetermined in the exact sence of having multiple individually sufficient explanations. The adaptive use that was originally primary may not remain so.

Accordingly, it is good practice when investigating an interesting behavior to presume that it is overdetermined and multiply adaptive — not to stop looking after the first sufficient explanation.

There's a sneaky way that this maxim can be interpreted as a definition: a behavior that is not overdetermined is probably not interesting.

Instead of asking “What is the purpose of this behavior?”, we should get used to asking ourselves “What is the collection of problems that this behavior solves?”.

Now consider human behavior in this light. Humor can establish social bonds, signal intelligence, negotiate status, relieve tension, and test social boundaries. Generosity can provide direct benefits, establish reciprocal obligations, signal wealth, advertise trustworthiness, and reinforce group cohesion. Intellectual curiosity can produce useful knowledge, provide entertainment, establish status, discover allies, and even serve as a means of courtship.

A rationalist who finds one plausible evolutionary explanation for any of these hasn't necessarily explained the behavior in full. They've identified one of its functions. And the functions need not be consciously represented. A person can be genuinely motivated by curiosity, while that curiosity also serves social and reproductive functions.

I will end this essay with a question for the reader to consider: What collection of problems does rationality technique solve?