Before we get into the Flynn effect and the steady increase (and possible subsequent decrease) in IQ scores across the world, we should probably talk about the elephant in the room: there are all kinds of problems with IQ tests.
The tests, created in 1905 by psychologists Alfred Binet and Théodore Simon, were originally intended as a way to find out which children needed extra attention paid to them in school. Towards the end of the 19th century, scientists believed that demonstrable abilities like verbal reasoning and working memory were indicative of underlying intelligence. Binet and Simon made a test to measure these skills, alongside others like visual-spatial skills, which would become the basis of the IQ test.
The test divided the child's score by their age and multiplied by 100 in order to provide them with their score. Now, IQs are standardized so that the average score is always 100, with numbers above or below this indicating that you scored higher or lower than that average.
"Simon and Binet thought the skills their test assessed would reflect general intelligence. But both then and now, there’s no single agreed upon definition of general intelligence," Dr. Stefan Dombrowski explained in a Ted-Ed Talk. "And that left the door open for people to use the test in service of their own preconceived assumptions about intelligence".
The real problem came when IQ was adopted by, you guessed it, eugenicists and the US military. The US military implemented mass IQ testing of recruits during World War I. The tests, which were used to determine who should be put forward for officer training, were used by eugenicists to make incorrect claims about intelligence being linked to race. In reality, US army recruits came from a diverse background and many were recent immigrants who were lacking in formal education, or fluency in English. Surprise surprise, if you lack these things you will perform worse on an IQ test, though you may be intelligent regardless.
Nevertheless, the tests are still considered useful as a way of measuring general intelligence. One way they are particularly useful is how the tests themselves haven't changed much over time. Though of course some adjustments have been made (more on that in a second), the consistency of the tests and the willingness of generations to subject people to tests over the decades mean we can measure how performance on those tests changed over time.
Putting the "intelligence is inherited" argument to bed, the results have shown an interesting increase over the 20th century, known as the "Flynn effect". Put simply, IQ points generally increase by about three points per decade. If you were to get the same result on the test as your father or grandfather did decades ago, this means you likely scored several points higher on the actual test, but your score is compared to a higher average.
So, what's causing this strange increase over the years (and, spoiler alert, a subsequent reversal)? Well, the Flynn effect put to bed the idea that intelligence was inherited. Unless there is some sort of secret eugenics program that has been working in the background to pass on intelligence genes by controlling human mating (you there, reaching for the tin foil: no there isn't) there must be other factors that led to this increased performance on IQ tests.
The effect can largely be explained by environmental factors, from improved schooling and emphasis on taking tests, to better nutrition over the years since the first and second World Wars. The problem is unpicking all the different possible causes, with better nutrition being a bigger factor affecting lower income groups, who also happened to be bigger beneficiaries of improved education during this time (given that they started from a lower base).
Of the most depressing explanations relates to lead. One study found that lead, for years found in gasoline, caused a drop in IQ levels, leading to a jump of about four or five points as lead was reduced and then removed in the 1980s.
The Flynn effect, unfortunately, may have its limits, and humanity is not just going to get marginally smarter every decade until we are all just massive brains in vats sat around discussing episodes of Frasier. A study of Norwegian conscripts, who were measured using IQ tests for half a century, has shown a decline in IQ from the mid-1990s. An Australian test on children has shown no improvement in IQ scores between 1975 and 2003. Tests in the UK have shown a drop of about 2-6 points (depending on age group) between 1980 and 2008.
Part of the reversal could be due to less emphasis on taking these tests and training for them, as well as the changes to social class over this time period. However, the World Health Organization and the Forum of International Respiratory Societies' Environmental Committee also proposed a possible explanation for the decline: a corresponding increase in pollution.
"Air pollution can damage the developing brain, which is especially concerning because this damage can impair cognitive function across the life span," a report into air pollution and noncommunicable disease states. As well as increasing risk of dementia, they cite a study conducted in Mexico City which found children living in more polluted areas had worse cognitive performance than their peers, adding "many studies have found that prenatal and early childhood exposure to PM2.5 is associated with delayed psychomotor development and lower child intelligence".
The good news, they say, is that air pollution can be reduced, and with almost immediate benefit to health. One weird one could be putting intelligence (as measured by IQ) on the upward trend once more.