The word “Intelligence” comes with a lot of baggage. In particular, any attempts to discuss a quantification of intelligence are often met with healthy skepticism, with claims like “There isn’t any one intelligence”. I myself also thought the same, that IQ tests were quite nebulous attempts to quantify with a single number something that was quite a multidimensional property. However, reading Stuart J. Ritchie’s fantastic primer on the subject, Intelligence: All That Matters brought home the point that the reason I thought like that was that all I had every read on the subject were some vague newspaper articles, and I had never actually read anything from an active researcher in the field. Intelligence is a fascinating brief primer on the subject of intelligence research, going through what intelligence is, where it comes from, how it is tested for, why it matters, and how it can be changed. Ritchie is a Postdoctoral Fellow in Cognitive Aging at the University of Edinburgh, whose primary research focus is studying Human Intelligence, so when writing about this, he knows what he is talking about.

While coming up with a snappy definition for intelligence is hard, Ritchie states that intelligence is nevertheless well measureable by scientific approaches. Over the past ~250 years, multiple scientists, including Darwin’s cousin Sir Francis Galton have come up with approaches for measuring and quantifying intelligence. Unfortunately, some of these attempts were tied to approaches to eugenics, leading to abhorrent consequences. But Ritchie argues for the separation of the understanding of the biological basis of intelligence and intelligence differences, from any political implications that may have been drawn from it.

An intelligence test currently tests reasoning, memory, knowledge, processing speed, and spatial ability, and multiple factors contributing to intelligence can be combined into a single g-factor, which measures a sort of general intelligence. Perhaps the most important takeaway from the book for me, which I realized I really had no idea about, was on why intelligence matters. Research has shown moderate to high positive correlation of IQ with longevity, educational attainment, and job performance. IQ is not the only factor that determines any of these results, and the correlation is far from perfect, but it has been shown to be much better than random.

Ritchie outlines how evolutionary pressures and genetic differences coutribute to increased intelligence in humans. Intelligence is a polygenic trait, i.e. multiple genes together contribute bits towards making up the total intelligence, rather than one (or a few) intelligence related genes. However, based on comparing identical and fraternal twins, a total of 50% of intelligence variability can be attributed to genetics. However, there is a lot still not well understood about the role of genetics, as well as brain size in determining intelligence.

Many things are claimed to increase IQ, including music and certain kinds of games. Ritchie outlines that the results on these claims are currently inconclusive. Improving health and education, however, have clearly been shown to improve IQ. The Flynn Effect shows that the population averages for IQs have been slowly but steadily increasing over time for the past few decades. (However, a recent study, which Ritchie discusses here suggests that the Flynn effect may be slowing down or reversing).

Ritchie concludes by outlining why intelligence research is so controversial, and a lot of points discussed here, like the historical connections to eugenics, the desire for equality between people, the complicated possibilities of sex and race differences (for which it is hard to unentangle genetic vs social causes), offer great food for thought.

All in all, this is a very fascinating, and a very quick read for a brief overview of the current state of intelligence research, what it actually measures, what that means for us. Ritchie outlines the current consensus in the field with great clarity, including pointing out few instances where people still differ and the science may not entirely be settled yet. Do check it out if you are interested in learning more about intelligence.