In terms of determinants of intellectual development, closely related people out to have more similar IQs than distantly related people, even when they are reared separately. Based on the finding of studies of heredity and intelligence, the average correlation for MZ twins reared together is .87, and .67 for those raised apart. These findings are a result of:

Cross-sectional research method

Explanation:

The Cross-sectional research method is a method of observational study design. Here, in this study the experimenter or the researcher measures and studies the outcome and and exposures in the study participants in the same time.

In the context, An experimenter studies the heredity and the intelligence of the MZ twins who are reared together and for those who are reared apart. The findings of the experiment made by the researcher is a result of a cross sectional research method in this case.