Fully half of the people on Earth are of below-average intelligence.
Think about it. In a group of 100 people, 50 of them will be less intelligent than everybody else.
Out of 100 freeway drivers, 50 of may not be smart enough to handle a roadside emergency.
Out of 100 politicians, 50 of them may not understand exactly what they are voting on.
Out of 100 university students, 50 of them may be taking liberal-arts courses on athletic scholarships.
And so forth...