The reasons this is true are not entirely clear. Does racism attract the unintelligent or do the unintelligent default into racist mentalities?
Studies going back over 50 years have repeatedly arrived at the same conclusion — racists have lower IQs than non-racists. The average intelligence quotient (IQ) of all members of the human race is 100 on the Stanford-Binet scale, as illustrated in the bell curves in the figure below. The average IQ of racists is up to 4 IQ points less than this.