Average IQ

An IQ test is normed: it is first given to a large sample of people, and their scores set the scale. The middle score of that sample becomes 100, and the spread is set so the standard deviation is 15 (Neisser et al., 1996).

So an IQ is always a comparison with the people a test was normed on. An IQ of 100 means you scored in the middle of them. The percentile says the same thing as a share: 100 is the 50th percentile. More on that: the IQ scale.

The Wisconsin Longitudinal Study has followed people who finished high school in Wisconsin in 1957, and many of their brothers and sisters, for most of their lives. Using its data, Hauser (2002) looked at the IQ they had as high school students next to the jobs they went on to do. The middle IQ in each job:

1. Cook92
2. Carpenter94
3. Electrician98
4. Police officer101
5. Nurse107
6. Primary school teacher109
7. Software developer110
8. Lawyer113
9. Doctor120

These are middle scores. Hauser found the link between job and test score only moderate: the scores inside each job spread widely, and about as widely in the jobs with the highest middle scores as in the others. Knowing someone's job tells you little about their own IQ.

Through the 20th century, people did better and better on IQ tests. In 1987, James Flynn collected data from 14 nations and found gains in every one of them, from 5 to 25 points in a single generation, with some of the largest on reasoning tests that need little reading or knowledge. The rise is now called the Flynn effect.

The largest review so far, Pietschnig and Voracek (2015), pooled 271 samples with almost 4 million people from 31 countries, tested between 1909 and 2013. Full-scale IQ rose by about 2.8 points per decade. Gains were biggest on reasoning puzzles, like the matrices in our test, and smaller on knowledge. They also found the gains getting smaller in recent decades.

In some countries the rise has turned around. Norway tests almost every young man at military conscription. Using those scores, Bratsberg and Rogeberg (2018) found that scores rose for men born up to the mid-1970s and then fell. The fall shows up between brothers in the same family too, which points to changes in the environment, not in who was having children.

This is why IQ tests are normed again from time to time. A test normed decades ago compares you with people who got fewer answers right, and gives a higher IQ than a test normed today.