The USC RFAB twin study uses a prospective, longitudinal design in which both monozygotic (MZ) and dizygotic (DZ) twins who have been studied approximately every 2–3 years beginning from age 9–10. The RFAB uses a comprehensive multi-informant, multi-measure approach with a key focus on externalizing psychopathology and its biological and social risk factors. An overview of the study design is presented in , which indicates the ages at which data are available, and the major areas of assessment, including anti-social outcomes as well as four major domains of risk factors (psychosocial, psychophysiological, neurocognitive, and personality). Assessments are made using three informants — twins provide self-report as well as ratings of their co-twins, while the twins’ primary caregiver (the biological mother in over 92% of families) and teachers also provide ratings of each twin. Official records are obtained from schools regarding both academic performance and discipline, and criminal justice records will be sought as the twins reach adulthood. To date, twins in the RFAB study have been assessed on at least one of five occasions, including four waves which are complete (Wave 1: age 9–10 years, 614 pairs; Wave 2: age 11–13 years, 445 pairs; Wave 3: age 14–15 years, 604 pairs; and Wave 4: age 16–18 years, 504 pairs), as well as an ongoing assessment (Wave 5: age 19–20 years, with a target sample of 625 pairs).
The USC Longitudinal Twin Study of Risk Factors for Antisocial Behavior (RFAB): Design, Constructs and Informants
The RFAB participants include 780 sets of twins and triplets (N
= 1,569 subjects), with males and females represented in approximately equal numbers. Both same-sex MZ and DZ pairs as well as opposite-sex DZ pairs are included. Zygosity of same-sex pairs was determined using DNA micro-satellite analysis, which measures concordance of DNA markers to assess whether DNA is identical (any pair with a discordant marker was to be DZ, while any pair with 7 concordant markers was considered to be MZ). In pairs where results were inconclusive (usually due to weak signal), zygosity was assessed with the Twin Similarity Questionnaire (Lykken, 1978
), a measure found to be 90% concordant with DNA zygosity in cases for which both measures were available (Baker et al., 2006
The sample is representative of the ethnic and socioeconomic diversity of the greater Los Angeles area (Baker et al., 2007
), with 28,6% Caucasians, 34.3% Hispanics, 13.1% Blacks, 4.1% Asians, 0.1% Native Americans, 17.6% mixed, and 2.2% other or unknown.
The numbers of families who participated in each completed wave (1–4) are presented in , along with expected N
’s for ongoing Wave 5. Of the 614 original Wave 1 families, 72% (n
= 445) returned for Wave 2, 71% (n
= 438) returned for Wave 3, and 59% (n
= 365) returned for Wave 4. While these retention rates for individual follow-up waves are modest (albeit quite favorable for a diverse urban sample and comparable to similar twin studies (Kendler et al., 2009
), the overall retention rate for original Wave 1 families returning to any
subsequent wave (including 2, 3, and 4) is quite favorable: 529 out of 614 (86%) of original Wave 1 families participated in at least one subsequent wave to date. The twin sample at Wave 5 will entirely consist of previous participants in the RFAB, drawn from the total longitudinal sample of 780 families (i.e., 614 original families who participated at Wave 1, plus 166 new families recruited at Wave 3). We have current contact information on >85% of the total pool of participants.
Number of Families of Twins Participating in Waves 1–4, and Projected for Wave 5
In addition to using three informants, that is, twin self-report, co-twin report, and parent ratings, we also obtain school records for each twin, including both academic performance (e.g., standardized achievement test scores) and behavioral (i.e., discipline) data. The assessments of externalizing psychopathology in this study are summarized in , indicating key constructs and informants used in each case. Social and biological risk factors for externalizing psychopathology are also assessed during each wave, using both surveys and laboratory tasks. These are summarized in . The longitudinal data on measures of anti-social behavior (ASB), including psychopathic personality, rule-breaking behavior and aggression, as well as psychiatric symptoms, autonomic measures of arousal and reactivity, are particularly represented across multiple (at least three, and in most cases five) time points. The only measures for which there are only two repeated observations are those involving electrocortical measures (electroencephalogram (EEG) and event-related potentials (ERPs)) and IQ, which are obtained at Waves 1 and 5.
Key Psychopathology Measures in Twins in USC-RFAB Across Waves 1–5
Measures of Social and Biological Risk Factors for Externalizing Behavior Problems of Twins in USC-RFAB across Waves 1–5
Attrition Analysis and Efforts for Tracking and Retention
Attrition analyses were carried out to examine whether those who discontinue participating during Waves 2, 3, or 4 differ from responders on baseline measures at Wave 1. Logistic regression analyses showed non-significant odds ratios (OR) for family socio-economic status, based on the Hollingshead Four-Factor index of Social Status (Hollingshead, 1979
; OR = 0.99, 95% CI, 0.98–1.01), twin’s gender (OR = 0.79, 95% CI, 0.59–1.07), interview language (OR = 1.13, 95% CI, 0.76–1.69), nor for ASB (Child Psychopathy Scale (CPS; total score; Lynam, 1997
) OR = 1.00, 95% CI, 0.97–1.03), reactive-proactive aggression total score (Raine et al., 2006
; OR = 1.00, 95% CI, 0.98–1.02). However, responders and non-responders were significantly different in ethnicity (OR = 0.70, 95% CI, 0.50–0.99), indicating that Caucasians were slightly less likely to drop out. Apart from the slight ethnic difference, those who discontinue participating in our study do so in a random manner.
To stay in contact with participating families we send out newsletters, annual birthday cards, and email personalized notecards to remind participants of upcoming lab visits. We also use various tracking procedures, including telephone number searches, returned mailings’ forwarding address information, and utilizing popular media (e.g., searches of Facebook and MySpace). We also periodically request that they verify/update current contact information. Additionally, we establish communication with their close friends and family members to learn their whereabouts if we have lost contact with them over time. Families are also invited to attend organized events of interest to twins, including a recent panel discussion on twins by top experts in the area of twins’ development, genetics, and psychotherapy, which helps create an atmosphere of inclusion. We provide each twin with a list of phone numbers and Web sites of agencies they can turn to for help. Finally, we offer a small monetary incentive for referrals.