The Flynn Effect
People are getting smarter, as IQ tests have to be adjusted. Since 1930's, when tests were introduced, more people end up in the higher tiers, so they have to be changed to standardise the results. Similiarly, when modern subject take older tests their score is higher.
This is possibly due to increasing complexity, which leads to an increase in intelligence. There's more information to process all the time.
Or it is possible that people are getting used to the tests.
It's also disputed and may not actually be an effect.