A reader writes:

I was reading your writings about mind and brain, and I was wondering about how IQ relates to all of this. Since IQ seems to have a large heritable component to it, and the only thing that can be inherited genetically is physical traits, does IQ and its heritability pose a threat to mind-body dualism?

It seems to me that someone with an IQ of 75 would have a very different mental experience than someone with an IQ of 145, and that they would also make decisions very differently, which, to me at least, would pose a threat to free will as well, since wouldn’t a certain level of intelligence be required to make decisions freely in a meaningful way?

It’s a very thoughtful question and an excellent point. The widely accepted heritability of IQ — between 57% and 80% in twin studies — is strong evidence for the materiality of the intellect. If intellectual activity is passed from generation to generation by DNA, then the capacity for abstract thought would seem to be material, and not immaterial nor spiritual.

However, if we adopt the traditional approach to the soul taken by Thomas Aquinas (1225–1274), the inheritance of IQ is quite consistent with the immateriality of the intellect and will. For one thing, much of IQ is heritable, but not all. The correspondence is not 100% — the child of two geniuses is not necessarily a genius.

Following the ancient Greek philosopher Aristotle, St. Thomas understood the human soul as a rational soul. On that view, our souls have three types of powers — vegetative, sensitive, and rational powers.

Vegetative powers are basic physiological functions — respiration, circulation, reproduction, etc.

Sensitive powers are the powers of sensation (vision, hearing, etc.), perception (the conscious experience of sensations), locomotion, imagination, memory, and emotions.

Rational powers are abstract powers — intellect (i.e. capacity for abstract reason) and will (i.e. free will).

Vegetative and sensitive powers are material powers caused by the brain — breathing, vision, memory, etc., are obviously caused by the matter of the body, brain, nervous system, etc.

Rational powers—abstract reason and free will—are not caused by the body or brain—they interact with matter but are not themselves generated by matter. They are immaterial powers of the human soul.

However, and this is the important point, the intellect and will are dependent on the sensitive and vegetative powers for their normal function. If you can’t breathe, you can’t think abstractly very well. If you heart doesn’t beat, your free will is quite impaired. If you can’t see, you can’t learn abstract ideas from books. If you can’t hear, you can’t learn abstract concepts by listening to someone. If you have a bad memory, you can’t learn abstract concepts very well because you can’t remember the perceptions and images necessary to evoke abstract thought.

In Aquinas’ terms, the vegetative and sensitive aspects of the soul “present” images to the intellect. The intellect, in turn, extracts the abstract concepts from the images and contemplates them using rational powers that are not generated by the brain itself. In this sense, the human soul is a composite of material and immaterial powers. When you read this post, your immaterial intellect abstracts the ideas presented here from the images that your material eyes and brain provide of the words you see.

Please remember that, in the approach to psychology offered here, perception, imagination, memory, and emotions are material powers, not immaterial powers. Knowledge comes into the immaterial intellect through material powers of vision, hearing, perception, imagination, memory, and emotion. So, for your normal immaterial intellect and will to work optimally, your material organs— brain, eyes, and ears—must work optimally.

For example, your understanding of calculus — a prime example of abstract immaterial thought — is highly dependent on the material powers of your brain. These material powers include your vision (to read textbooks), your hearing (to hear your professor), and your memory (to remember what you read or heard yesterday).

This distinction highlights the fundamental difference between human beings and nonhuman animals — humans have an immaterial aspect of the soul (intellect and will) that animals lack. Animals, who are wholly material creatures, have memories, emotions, and the capacity to form perceptual images. But they can’t reason abstractly — my dog thinks of her food in her dish, but she doesn’t think about nutrition.

If we make this distinction between the material and immaterial powers of the human soul, the heritability of IQ can be better understood. What is heritable about IQ is not intellect and will but the capacity for perception, imagination, memory and emotion. Suppose you have very good (material) perception and imagination (i.e. you can see well, hold images in your head, and manipulate them mentally with great skill). Perhaps you also have a very good memory, plus healthy emotions (the emotional maturity and stability to study and seek knowledge consistently). Then you can present your abstract intellect with really good information.

If you lack these material powers of perception — imagination, memory, and emotional stability — you can’t form images and memories in your mind efficiently and effectively. Thus you can’t present these images and memories to your abstract intellect in a way that helps you understand them.

So IQ depends on many things — on your vision and hearing, your imagination, memory and emotions, as well as on your intellect and will.

What is heritable in IQ is the material powers of the soul. Good perception, good imagination, good memory and good emotion make your immaterial intellect and will more effective.

Put another way, effective material powers of the soul (which are heritable as IQ) are necessary but not sufficient for effective exercise of immaterial intellect and will.

If you would like a deeper understanding of the Thomistic understanding of the mind brain relationship, philosopher Edward Feser has an excellent book, Aquinas: A Beginner’s Guide, on philosophical issues as seen from the Thomistic perspective. Chapter 4 addresses these topics in detail.

You may also wish to read:

Are we humans getting smarter or have we peaked? The really surprising thing, science writer David Robson notes, is that it may not matter as much as we think. There is no clear evidence that key thinking skills improve with measured intelligence.

Why intelligent women marry less intelligent men Are they trying to avoid competition at home as well as at work? Or is there a statistical reason we are overlooking? Psychological theories abound but the true explanation is a statistical one: Regression to the mean. It also applies to many other choices in life. (Gary Smith)

and

Why a budding neuroscientist is skeptical of brain scans. After reading her perceptive essay about the problems in fMRI imaging in neuroscience, I’m sad that a gifted student has doubts about a career in the field. Neuroscience badly needs skeptics to show how unreliable technology, biased handling of data, and materialism’s conceptual mess frustrate science. (Michael Egnor)