6 is a lot for an average, then again 23 is a small n
 
http://www.mdpi.com/2079-3200/5/1/1/htm
 
https://en.wikipedia.org/wiki/Minnesota_Transracial_Adoption_Study
 
https://www.infona.pl/resource/bwmeta1.element.elsevier-a6ad7a0a-260b-378d-9df6-f9d1e70b82ad
 
Follow-up is even lower. And they go from a minimum of 92 to 62. Meaning some retards were there.
 
Table 2 was really small n's
 
Of course it is bigger than the other transracial adoption studies and is more hereditarian, they needed to contrive attrition