The Test That Predicted Road Deaths Better Than Medical History

A study of more than forty-six thousand men, tracked over years, compared 57 different variables to predict which of them would die before turning thirty-five. Only one came out on top: a mental test that had nothing to do with health.

It wasn’t medical history.

It wasn’t behavioral record or education level, even though those were measured too.

It was a score on a mental aptitude test, taken years before any accident, that best predicted who would still be alive.

A study at an unusual scale

Psychologist Arthur Jensen documents in The g Factor (1998) a piece of research by investigator B. J. O’Toole, published in 1990, with a sample size few psychological studies ever reach.

• The cohort included 46,166 men, aged twenty to thirty-four, who had previously served in the Australian armed forces.
• Of that group, 523 had died from causes unrelated to combat by the time of the study.
• All of them had a score on the Army General Classification test (AGC), a heavily g-loaded test, along with other tests of speed, accuracy, and mechanical comprehension.

Alongside those scores, researchers collected 54 additional variables, grouped into three broad categories: education level, personal conduct (alcohol use, arrests, unauthorized absences from military service), and health history (number of days hospitalized, among other medical data).

57 variables entered the race, and only one made it to the finish line

The statistical analysis, a multiple regression with all 57 variables at once, set out to answer one very specific question: of all of them, which ones independently predict who would still be alive years later?

The result, as Jensen describes it, was decisive: the AGC, the most heavily g-loaded test administered, was the only variable that made a statistically significant, independent contribution to predicting mortality.

A one-standard-deviation increase in AGC score was associated with a 16% reduction in mortality rate.

And here’s the study’s most uncomfortable detail: each person’s health history, with all its real medical data, turned out not to be as good a predictor as that mental test taken years earlier.

Most of those deaths had nothing to do with health

The reason for this seemingly strange result becomes clear once you look at the cause of those deaths.

Most of the deaths in that age group (twenty to thirty-four) were due to external causes, mainly traffic accidents, not illness. A specific analysis of motor-vehicle-accident mortality produced this table, organized by IQ level:

IQ Level	Death rate per 10,000
Above 115	51.3
100 to 115	51.5
85 to 100	92.2
80 to 85	146.7

The traffic-accident death rate nearly triples between the highest and lowest IQ groups. It isn’t an illness that explains the difference, it’s something related to behavior in fast-changing, high-risk situations, like driving.

Why a mental test predicts better than a medical report

The explanation that emerges from the book isn’t that IQ “protects” against risk in some magical way, but that many of the decisions that determine whether an accident happens (assessing a traffic situation in split seconds, anticipating another vehicle’s behavior, reacting to the unexpected) depend, in part, on cognitive processes that the g factor captures well and that a medical report, focused on illness and hospitalization, doesn’t capture at all.

Jensen extends this idea to other areas: in the same chapter, he notes that numerous social problems (dropping out of school, chronic dependence on welfare, delinquency, poverty) show an inverse relationship with IQ, independent of socioeconomic origin, occurring up to five times more often in the lowest IQ quartile than in the highest.

The limits of a finding this striking

Before drawing any conclusion about a specific person, it’s worth being very precise about what this study allows us to say, and what it doesn’t:

• It’s a difference in rates between groups of tens of thousands of people, not an individual prediction.
• The vast majority of people, at any IQ level, don’t suffer a serious accident.
• One specific person’s real risk depends on countless factors (driving experience, mood, road conditions, luck) that this study, focused on population trends, can’t capture.

What this finding does leave behind, uncomfortable and fascinating in equal measure, is an idea rarely thought of this way: part of what keeps us safe in the world has less to do with the body we bring to the doctor, and more to do with how quickly and precisely that same body processes what’s happening around it.

If you’re curious how the ability to process patterns quickly under pressure gets built, chess grandmasters don’t have a better memory than you explains what really sets an expert apart from a beginner.

Sources

• Jensen, A. R. (1998). The g Factor: The Science of Mental Ability. Praeger.
• O’Toole, B. J. (1990). Intelligence and behavior and motor vehicle accident mortality. Accident Analysis and Prevention, 22, 211-221.