When ethnic groups intermarry, it is likely to result in cognitive benefits; indeed, perhaps already having contributed to the Flynn effect, which has been the worldwide rise in average IQ score by as much as three points per decade over most of the past century.

A) When ethnic groups intermarry, it is likely to result in cognitive benefits; indeed, perhaps having contributed to the Flynn effect, which has been : Dangling modifier. Not sure to what it is referring to.

B) The intermarriage of ethnic groups is likely to result in cognitive benefits and may indeed have contributed to the Flynn effect,

C) When ethnic groups intermarry, they are likely to result in cognitive benefits and may indeed have contributed to the Flynn effect, the name given to : Change in meaning + "the name given" is wordy
D) Ethnic groups that intermarry will likely result in cognitive benefits; indeed, possibly having contributed to the Flynn effect, : Same as A

E) The intermarriage of ethnic groups, likely to result in cognitive benefits, indeed have possibly contributed to the Flynn effect, the name given to : Same as A
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