Simplify the following text for a Grade 11 native English-speaking person.
Original: The average IQ scores for many populations have been rising at an average rate of three points per decade since the early 20th century with most of the increase in the lower half of the IQ range : a phenomenon called the Flynn effect .
Simplified: The average IQ scores for many populations have been rising at an average rate of three points per decade since the early 20th century , a phenomenon called the Flynn effect .