What Is Intelligence? Not Even the Experts Agree

They’ve been at it for more than a century. Entire university departments are devoted to it. And yet experts on intelligence can’t agree on something as basic as what exactly it is they’re studying.

It isn’t for lack of research.

It isn’t for lack of interest.

It’s that the word “intelligence” itself, as used in everyday language, turns out to be surprisingly slippery the moment anyone tries to pin it down seriously.

Almost as many definitions as experts

Psychologist Arthur Jensen sums it up in The g Factor (1998) with a line that leaves little room for ambiguity: “no other term in psychology has proved harder to define than ‘intelligence.’” And he adds something even more striking: there are almost as many different definitions of ‘intelligence’ as there are experts who’ve tried to define it.

It isn’t that each expert proposes a slightly different definition of the same idea. It’s that they focus on genuinely different aspects:

• Some define it around the capacity to reason and solve novel problems.
• Others, around the capacity to adapt to changing situations.
• Others, around the speed or efficiency of learning.
• Others include the capacity for abstraction, or handling symbols, or long-term planning.

Each of these proposals is reasonable on its own. The problem is that, together, they don’t outline a single concept, they outline several, partially overlapping, with no shared criterion for deciding which one is “the” correct definition.

Spearman’s solution: stop arguing over the word and measure the correlation

In the early 20th century, British psychologist Charles Spearman made a methodological decision that would change the course of the entire field: instead of continuing to argue over what “intelligence” is in the abstract, he set out to observe something far more concrete and measurable.

Spearman started from an empirical finding that earlier thinkers like Herbert Spencer and Francis Galton had already sensed: scores on very different mental tests (vocabulary, numerical reasoning, spatial orientation) tend to be positively correlated. Whoever scores high on one tends to score high on the others, even when they seem, on the surface, to have nothing in common.

To put this to a rigorous test, Spearman invented factor analysis, a statistical method capable of determining how much each of those tests relates to an underlying common factor. That common factor, present in some form in any task that demands mental effort, he simply called g (for “general”).

An anecdote that sums up the idea better than any technical definition

To illustrate this same intuition long before factor analysis existed, Jensen recounts an anecdote involving writer Samuel Johnson (1709-1784).

A well-known historian remarked to Johnson that it was thanks to very different talents that Caesar became a great commander, Shakespeare a great poet, and Newton a great scientist. Johnson’s reply was blunt:

“No, it is only that one man has more mind than another. He may direct it differently, or prefer this study to that. Sir, the man of vigor may walk to the north as well as to the south, to the east as well as to the west.”

Two centuries before Spearman, Johnson had already sensed, in a single line, what factor analysis would eventually prove with numbers: that behind seemingly unrelated talents like military leadership, poetry, and science, there could be something in common, a kind of general mental “vigor,” applicable in any direction.

Why this isn’t just an academic word game

It might seem like swapping “intelligence” for “the g factor” just replaces one hard-to-define word with another equally abstract one. That isn’t the case, and the difference matters:

• “Intelligence” is a word from everyday language, loaded with different connotations depending on who’s using it.
• “g” is a statistical factor, defined operationally from real correlation patterns between tests, not from a prior intuition about what being intelligent “should” mean.
• This makes possible something the philosophical debate over the word “intelligence” could never offer: measuring how much of that common factor each specific test reflects, comparing tests against each other, and accumulating verifiable empirical evidence about its biological correlates and its role in real-world outcomes.

Spearman didn’t resolve the philosophical debate over what “intelligence” is. He sidestepped it, wisely, and proposed instead a question that data could actually answer: is there a factor common to all mental abilities, and how can it be measured?

What’s left once you stop arguing over the word

More than a century after Spearman, the word “intelligence” still has no single definition accepted by all experts. And it probably never will, because it’s, above all, an everyday word, not a technical term with precise boundaries.

What does exist, with a century of accumulated research behind it, is a well-characterized statistical factor, with measurable biological correlates (from the brain’s energy efficiency to reaction time in milliseconds) and a demonstrated ability to predict outcomes in fields as different as academic performance and road safety.

We may never agree on exactly what it means to be “intelligent.” But that, as Spearman showed more than a century ago, was never really the question that needed answering.

If you want to see that same g factor in action, measured in fractions of a second, what your brain decides in a fifth of a second explains the experiment that’s been putting it to the test for more than a century.

Sources

• Jensen, A. R. (1998). The g Factor: The Science of Mental Ability. Praeger.
• Spearman, C. (1904). “General intelligence,” objectively determined and measured. The American Journal of Psychology, 15(2), 201-292.