Diachronic vs. synchronic explanations: Why psychology needs both

So I'm at Harvard now, and settling in fine. I don't have internet at home yet, so posting might be a little bit spotty until I do. Now, let's jump into it.

Steve Pinker's "How The Mind Works" is a pretty classic book at this point. New York Times bestseller, Pulitzer finalist, and one of the few real, undiluted and scientific psychology/cognitive science books to have made it into the public consciousness. Pinker wears somewhat different hats in each of his books, and in this one, he is a staunch evolutionary psychologist -- dismissing Lewontin and Gould's "spandrel" argument out of hand (and unconvincingly, in my view), and claiming that psychology must turn to evolution for answers to its important questions, How The Mind Works.

The philosopher and cognitive scientist Jerry Fodor wrote a book titled "The Mind Doesn't Work That Way", both an obvious jab at Pinker's lofty title, and a serious critique of his ideas. One of Fodor's chief arguments is against evolutionary psychology as a whole. The argument can be summarized like this:

How something came to be is a "diachronic" (across time) explanation, and how something is now is a "synchronic" (concurrent time) explanation. According to Fodor, diachronic and synchronic explanations are unrelated to each other, because they answer different questions. The former answers the question of "How X came to be" or "Why X is the way it is, and not some other way". The latter answers the question of "How X works". Furthermore, Fodor argues that the twain shall never meet. Diachronic investigations are, in principle, incapable of answering how something works, and synchronic explanations are, in principle, incapable of answering how something came to be.

An illustrative analogy is the study of flight and the building of airplanes. If you want to know how to build an airplane, you don't go to the museum and study the history of flight, tracing its progress and evolution from the Wright brothers on. You ask an engineer and get the blueprints for a real, functioning, modern airplane. If you don't have access to that sort of thing, and we don't when it comes to the mind, you try to reverse-engineer a modern airplane. The history of flight, in short, will not help you.

What this all means is if we want to know how the mind works, evolutionary explanations (which are diachronic) are unhelpful. If we want to know why the mind is the way it is, and not some other plausible way -- for example, when we've built a model that mimics a particular function, but doesn't seem to work the way the mind does, and we want to know why -- we can look into evolution. But if we want to know how the mind works in its present form, we need to study it directly. Good old psychology.

When I first heard about this argument, I wasn't just stumped, I was distraught. I'm not the biggest fan of evolutionary psychology, mainly because I think it has some serious problems in its execution and the methodology available to it. But neither am I willing to throw the whole enterprise out or concede that it's irrelevant. Before I had any good reason, I had a strong gut feeling that Fodor was wrong. The good reason took a couple of days.

Fodor's mistake is ignoring the real reason for the existence of evolutionary psychology: the way that something came to be constrains the range of possibilities for how it can be now. We may have the capacity to completely redesign an airplane if we discover a better way for it to be built, but evolution is famous for its inertia. As Pinker points out, the reason the male seminal ducts don't wind straight down, hooking around the ureter, is that our reptilian ancestors' testicles were inside their bodies, and when our bodies became too warm to produce semen and the testicles had to descend into the sack of their present form, evolution could not redesign the plumbing, and the seminal ducts got caught on the ureter "like a gardener who snags a hose around a tree".

Of course, in this case, we can simply look into a human male and see the design of the seminal ducts. The synchronic explanation is available to us. But imagine if it was not -- for example, if we lived in some universe where evolution had been discovered, but where, as in the middle ages, dissection of cadavers was forbidden and imaging technology did not exist. Wouldn't it help to know that our reptilian ancestor's testicles were inside their bodies, and that the ducts wound around the ureter? Wouldn't it help to know even more that non-human mammals, especially those who we share a close evolutionary ancestry with, and whose testicles did descend into an external sack have their ducts snagged on the ureter? Would it really remain unconvincing if we found this design in all mammals with descended testicles? Or would we then think it pretty likely that our bodies have the same design?

The issue is that, while synchronic explanations provide evidence through deductive reasoning, diachronic explanations are necessarily inductive. Barring extreme philosophical skepticism that would claim every bit of reasoning based on observation is inductive, if a dissection or an X-ray-type body image shows seminal ducts going around the ureter, then that is the direct conclusion we deductively draw from that information, coupled with our knowledge about the reliability of the technology used, its functioning, and the trustworthiness of our own senses. On the other hand, there is nothing as compelling about finding the snagged seminal ducts design in our closesy ape ancestors. It could be that some beneficial mutation did take place between them and us, so that the design of our male reproductive system is different. But the closer the ancestors where we find the same design, and the more the findings in other animals support the evolutionary hypothesis that predicts which animals will have the same design (that is, all of the ones with descended testicles that are known to have evolved from reptiles), the more confident we can be.

And in studying the mind, this is the kind of thing we have to do, because synchronic explanations are often unavailable. Medieval sensibilities aside, we simply don't have the tools to directly divine the exact design of our cognitive machinery (and that is still true, despite all recent advances in neuroimaging). Evolutionary psychology, if we are careful with it and remain mindful of its shortcomings, can provide inductive evidence about the design of some particular cognitive system, if we can more directly observe the antecedents of this design in creatures closely related to our own evolutionary ancestors of different times, and chart its evolutionary trajectory (what it looked like in fish, in reptiles, in mammals, in monkeys, in apes...) Evolutinary psychology isn't dead or useless. It might, however, be humbled. And given some of its more blatant trespasses in recent years, that's not a bad thing.