Studies of twins reared-apart and -together can untangle environmental effects further than classic designs. These reared-apart studies can partition the shared environment into the narrower rearing environment -- common only to twins reared together, and the broader correlated environment -- common to twins irrespective of rearing. Twins reared apart, however, are rare and arise from extraordinary circumstances. As a result, these studies have small samples that are unlikely to be representative or ethically replicated.
In the current study, we address those limitations with an extended family design based on ordinary kinship categories: siblings, cousins, parent-child, and aunt-nibling. Similar to the twins reared-apart and -together, the shared environment can be partitioned into a narrow rearing environment (r) that siblings share and a broader correlated environment (c’) that the extended family share. We call this model the AC’RE model.
We illustrate this design, using data from the National Longitudinal Surveys of Youth (NLSY). The NLSY is multigenerational study and is based on a nationally representative household probability sample from the United States. Recently developed kinship links have produced over 42,000 kinship pairs (Rodgers, et al 2016); we use 36,370 of them across the following groups:
- Monozygotic Twins (n = 29 pairs)
- Full Siblings (9688)
- Parent-Child (11477)
- Half Siblings (3390)
- Aunt-Nibling (7511)
- Cousins reared together (96)
- Cousins reared apart (4179)
We estimated the heritability of self-reported adult height (standardized within gender, grouped by minority status), using the AC'RE model. Estimates varied by minority status. For minorities, a2 was 80% (95% confidence interval [68, 93]), c'2 4.6% (95CI [.3, 14]), r2 .3% (95CI [0, 10]), and e2 was 15% (95CI [8.7, 24]). For non-minorities, a2 was 88 % (95CI [81, 96]), c'2 0% (95CI [0, 1]), r2 .3% (95CI [0,8]), and e2 was 11% (95CI [5.4, 19]). The model fit very well; RMSEA was .016 (90CI [0, .02]), TLI and CFI >.99, and SRMR was .034.
Estimates were consistent with the literature and suggest that shared-environmental sources of height among minorities arise from outside the home rather than from within it. We will highlight other applications of the AC'RE model, note the differing interpretations of the rearing environment, and discuss the challenges associated with modeling intergenerational effects.
References:
Rodgers et al (2016). The NLSY Kinship Links: Using the NLSY79 and NLSY-Children Data to Conduct Genetically-Informed and Family-Oriented Research. Behavior Genetics.
