This 2015 Virginia study used Swedish data to find:
“Adoption into improved socioeconomic circumstances is associated with a significant advantage in IQ.”
The study’s all-male subjects were in 436 sibling relationships:
“..in which at least one member was reared by one or more biological parents and the other by adoptive parents. IQ was measured at age 18–20 as part of the Swedish military service conscription examination.”
One of the researchers said:
“Environmental effects have to be inferred, as in the rare event when pairs of siblings are raised by different parents in different socioeconomic circumstances. The Swedish population data allowed us to find that homes led by better educated parents produce real gains in the cognitive abilities of the children they raise.”
Let’s approach this study from the adopted boys’ perspectives. Their biological families’ situations had to be hellishly tragic in order to separate siblings and put one of them up for adoption. I didn’t find at what age the separations typically took place, but can you imagine what the adopted child felt?
A child is very sensitive to his caregivers’ words, body language, facial expressions, physical touches – to all the things that show him he’s loved. A child learns at an early age from both implicit and overt expressions whether or not he’s accepted for who he is.
It’s extremely traumatic for a child to be rejected for who he is. Consider this passage from Dr. Arthur Janov’s book The Primal Scream:
“Parental need becomes the child’s implicit command.
The child is born into his parents’ needs and begins struggling to fulfill them almost from the moment he is alive.
He may be pushed to smile (to appear happy), to coo, to wave bye-bye, later to sit up and walk, still later to push himself so that his parents can have an advanced child.
As the child develops, the requirements upon him become more complex.
He will have to get A’s, to be helpful and do his chores, to be quiet and undemanding, not to talk too much, to say bright things, to be athletic.
What he will not do is be himself.”
All of the above can happen within a stable family. Can you imagine what a child in an unstable family felt as he learned he wasn’t accepted, and how he tried to adapt?
Everything these adopted children did to be accepted by their original caregivers failed. They were rejected by and ejected from the people who were supposed to love them!
Can you imagine how desperate these adopted children would have been in their new environment?
What wouldn’t they have done to be accepted?
The researchers made a point of cognitive development. But of all of the things that were important to the adopted child, that described his quality of life, does the finding of a higher IQ give even the slightest hint of his reality?
http://www.pnas.org/content/112/15/4612.full “Family environment and the malleability of cognitive ability: A Swedish national home-reared and adopted-away cosibling control study”