Contrary to popular belief (and what most social scientists think), all good science is reductionist. The particle physicist and Nobel laureate (and one of my intellectual heroes) Steven Weinberg expresses it best, when he says “The reductionist worldview is chilling and impersonal. It has to be accepted as it is, not because we like it, but because this is the way the world works.”
But why is it? Why is the reductionist worldview the way the world works? It’s very simple. In nature, bigger things are made up of smaller things. Smaller things are not made up of bigger things. So phenomena at a higher level of aggregation (“bigger things”) must be explained by causal mechanisms operative at a lower level of aggregation (“smaller things”). Humans are made up of genes and cells and proteins. Genes and cells and proteins are not made up of humans. So human behavior (phenomena at the level of humans) must be explained by mechanisms operative at the levels of genes, cells, and proteins. Of course, the behavior of genes, cells, and proteins must be explained at the levels of molecules, elements and, ultimately, elementary particles and, possibly, vibrating strings. Just like turtles, reductionism goes all the way down (but, unlike turtles, it stops at the fundamental constituent in nature, formerly believed to be the atom and currently thought to be the vibrating string).
It is important to point out, however, that reductionism does not mean that physics, the most fundamental of all sciences, can explain everything in nature all by itself. Higher-level, emergent phenomena require separate laws, in addition to more fundamental laws, for their explanation. In other words, reductionism does not mean that we can do away with higher-level, less fundamental, sciences, such as social sciences. Once again, Weinberg explains it perfectly in his 1992 book Dreams of a Final Theory:
I do believe there is a sense in which everything is explained by the laws of nature and the laws of nature are what physicists are trying to discover. But the explanation is an explanation in principle of a sort that doesn't in any way threaten the autonomy of the other sciences. We see this even within physics itself. The study of statistical mechanics, the behavior of large numbers of particles, and its applications in studying matter in general, like condensed matter, crystals, and liquids, is a separate science because when you deal with very large numbers of particles, new phenomena emerge. To take an example I have used elsewhere, even if you tried the reductionist approach and plotted out the motion of each molecule in a glass of water using equations of molecular physics to follow how each molecule went, nowhere in the mountain of computer tape you produced would you find the things that interested you about the water, things like turbulence, or temperature, or entropy. Each science deals with nature on its own terms because each science finds something else in nature that is interesting. Nevertheless, there is a sense that the principles of statistical mechanics are what they are because of the properties of the particles out of which bodies are composed. Statistical mechanics does not have principles that stand alone and cannot be deduced from a deeper level.
Similarly, laws of evolutionary biology alone cannot explain why some men become career criminals while others become law-abiding citizens, or why some marriages last forever while others end in divorce, let alone why wars and revolutions and economic recessions occur. In order to explain these macrolevel, emergent phenomena, one needs laws of sociology, political science, and economics. However, these additional laws of social sciences cannot be inconsistent with more fundamental laws of evolutionary biology; for example, a complete macrosociological theory of revolutions cannot contain an assumption which states that human actors equally value the welfare of their own genetic offspring and that of someone else’s offspring, or that men and women are equally predisposed to engage in physical violence. In other words, social sciences are to evolutionary biology what statistical mechanics are to elementary particle physics in the Weinberg quote above. To paraphrase Weinberg, social sciences do not have principles that stand alone and cannot be deduced from a deeper level.
Leda Cosmides and John Tooby, cofounders of evolutionary psychology, remind us that, before Galileo and Newton, the celestial science (about the motions of heavenly bodies) and terrestrial science (about the movements of objects on earth) were considered to be separate sciences, governed by separate sets of laws and principles. It was a huge step forward in the history of science to break down the wall of separation between them, as Galileo and Newton did, and to recognize that the same set of laws and principles applies to both celestial and terrestrial bodies.
That, unfortunately, is the sorry state of social sciences today. Social sciences in the 21st century are where physical sciences were in the 17th century. Social scientists believe in the firm separation between human sciences (social sciences) about the behavior of human species, and nonhuman sciences (biology) about the behavior of all other species in nature, governed by entirely different sets of laws and principles. It would be a huge step forward in the history of science to break down this wall as well, and subsume social sciences under biology.
I will conclude this post with another favorite quote of mine from Weinberg.
The reason we give the impression that we think that elementary particle physics is more fundamental than other branches of physics is because it is.
The reason we give the impression that we think that evolutionary psychology is more fundamental than other branches of social and behavioral sciences is because it is.