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Not only did the twins experience different cultures growing up, they also were raised in very different family environments. The twin who remained in South Korea was raised in a more supportive and cohesive family atmosphere. The twin who was adopted by the U.S. couple, in contrast, reported a stricter, more religiously-oriented environment that had higher levels of family conflict.

The researchers found “striking” differences in cognitive abilities. The twin raised in South Korea scored considerably higher on intelligence tests related to perceptual reasoning and processing speed, with an overall IQ difference of 16 points.