Actually yes, it's called "outbreeding depression". A white has an average IQ of 100, whereas an African's is around 70-80. So the half-cast individual has an IQ in the middle, around 85. Plenty of statistics to back this up. You can see this in african-americans who are around 15-30% european genetically. They have an average IQ around 10-15 points higher than full africans.