The Flynn Effect is the increase of intelligence test scores over the 20th century. The IQ measure is measured around 100 and their standard deviation on either sides. Over the years, the mean is adjusted based on the sample population and it is observed that the mean shifts upwards a few basis points.
There are a few explanations as to why this is a trend.
Total schooling time has increased steadily over the years. This means that kids would have had more chances to practice their analytical and cognitive skills.
Tests similar to IQ tests are used in a lot of scenarios. The familiarity of such tests has grown.
There is a more stimulating environment while kids are growing up now vs. back in the day. More video games and television stimuli. There are studies that show better cognitive activity and hand-eye coordination among those who game more.
Nutrition, both in quantity and quality has improved over time. There is data that suggests that the brain is ever so slowly growing as well with the improved conditions.
Better health conditions. Developed nations in general are more equipped to handle infectious diseases. Vaccinations are part of everyone’s growing life.