Child Development

Although the MSUTR initially began as a university-based twin registry assessing undergraduate men and women, we moved to recruiting twins via birth records beginning in 2004. The Michigan Department of Health and Human Services (MDHHS; formerly known as the Michigan Department of Community Health) identifies twin pairs residing in lower Michigan who meet our study age criteria (see criteria below) and whose addresses or parents’ addresses (for twins who are minors) can be located either using driver’s license information obtained from the state of Michigan or the proprietary search engine used by police (since the MDHHS, as the state agency in charge of vital records, has direct access to individual SSNs, full names, and birth dates). Twins are identified either directly from birth records or via the Michigan Twins Project, a large-scale twin registry within the MSUTR that doubles as a recruitment resource for smaller, more intensive projects. Because birth records are confidential in Michigan, recruitment packets are mailed directly from the MDHHS to eligible twin pairs to ensure their confidentiality. Twins indicating interest in participation via pre-stamped postcards or e-mails/calls to the MSUTR project office are then contacted by study staff to determine study eligibility and to schedule their assessments.
Four recruitment mailings are used for each study to ensure optimal twin participation. Overall, response rates across studies (56–85%) are on par with or better than those of other twin registries that use similar types of anonymous recruitment mailings, and have thus far yielded largely representative samples. Families recruited into the families of the naturally-conceived twins in the large-scale Michigan Twins Project, for example, closely resemble families across the state of Michigan (Burt & Klump, 2012 (link)). The proportion of MTP families that identify as White, non-Hispanic is 81.0%, which is very similar to the 80.2% indicated in state-wide Census data. Mean family incomes are also quite comparable ($75,940 in the MTP versus $73,373 in the Census), as are the proportion of families with graduate or professional degrees (10.3% in the MTP versus 9.6% in the Census).
These successes extend to our smaller, more intensive in-person studies as well. For example, the 529 population-based twin families and 502 at-risk twin families in the completed Twin Study of Behavioral and Emotional Development in Children (TBED-C) were generally representative of recruited but non-participating families. As compared to non-participating twins, participating twins reported similar levels of conduct problems, emotional symptoms, or hyperactivity (d ranged from −.08 to .01 in the population-based sample and .01 to .09 in the at-risk sample; all ns). Participating families also did not differ from non-participating families in paternal felony convictions (d = −.01 and .13 for the population-based and the at-risk samples, respectively), rate of single parent homes (d = .10 and −.01 for the population-based and the at-risk samples, respectively), paternal years of education (both d ≤ .12), or maternal and paternal alcohol problems (d ranged from .03 to .05 across the two samples). However, participating mothers in both samples reported slightly more years of education (d = .17 and .26, both p < .05) than non-participating mothers. Maternal felony convictions differed across participating and non-participating families in the population-based sample (d = −.20; p < .05) but not in the at-risk sample (d = .02). In short, our recruitment procedures thus appear to yield samples that are representative of both recruited families and the general population of the state of Michigan.

Participants were members of the Environmental Risk (E-Risk) Longitudinal Twin Study, which tracks the development of a nationally representative birth cohort of 2,232 British children (see Figure 1 for the geographical location of families living throughout England and Wales). The sample was drawn from a larger birth register of twins born in England and Wales in 1994–1995 (Trouton, Spinath, & Plomin, 2002 (link)). Details about the sample have been reported previously (Moffitt, 2002 (link)). Briefly, the E-risk sample was constructed in 1999–2000, when 1,116 families with same-sex 5-year-old twins (93% of those eligible) participated in home-visit assessments. Families were recruited to represent the UK population of families with newborns in the 1990’s, based on (a) residential location throughout England and Wales and (b) mother’s age (i.e., older mothers having twins via assisted reproduction were under-selected and teenage mothers with twins were over-selected). Follow-up home visits were conducted when the children were aged 7, 10, and 12 years and, with parents’ permission, questionnaires were mailed to the children’s teachers at each age. Herein we report on data from the Phase 12 assessment when 96% of the families participated, among which 94% were living in on a street captured by Google Street View (n = 1,012 families, n = 2,024 children). Parents gave informed consent and children gave assent. The Maudsley Hospital Ethics Committee approved each phase of the study.