rearing environment, but that MZ twins share all of their genetic variation, while DZ twins share half of their genetic variation, on average. Shared environmental influences refer to experiences or events that both twins experience that make them more similar (e.g., growing up in the same neighborhood). Non-shared environmental influences refer to experiences or events that one individual experiences, but not his/her co-twin (e.g., having different friends). When the MZ correlation for a variable is larger than the DZ correlation, this suggests that there are genetic influences. When the DZ correlation for a variable is approximately half of the MZ correlation or lower, this suggests that there are no shared environmental influences. Finally, when the DZ correlation for a variable is more than half of the MZ correlation, this suggests the presence of shared environmental influences.
