This review offers an update on research conducted with FinnTwin12 (FT12), the youngest of the three Finnish Twin Cohorts. FT12 was designed as a two-stage study. In the first stage, we conducted multiwave questionnaire research enrolling all eligible twins born in Finland during 1983–1987 along with their biological parents. In stage 2, we intensively studied a subset of these twins with in-school assessments at age 12 and semistructured poly-diagnostic interviews at age 14. At baseline, parents of intensively studied twins were administered the adult version of the interview. Laboratory studies with repeat interviews, neuropsychological tests, and collection of DNA were made of intensively studied twins during follow-up in early adulthood. The basic aim of the FT12 study design was to obtain information on individual, familial and school/neighborhood risks for substance use/abuse prior to the onset of regular tobacco and alcohol use and then track trajectories of use and abuse and their consequences into adulthood. But the longitudinal assessments were not narrowly limited to this basic aim, and with multiwave, multirater assessments from ages 11 to 12, the study has created a richly informative data set for analyses of gene–environment interactions of both candidate genes and genomewide measures with measured risk-relevant environments. Because 25 years have elapsed since the start of the study, we are planning a fifth-wave follow-up assessment.

Early maturation, indexed by pubertal development (PD), has been associated with earlier initiation and greater frequency of adolescent substance use, but this relationship may be biased by confounding factors and effects that change across development. Using a population-based Finnish twin sample (N = 3,632 individuals), we conducted twin modeling and multilevel structural equation modeling of the relationship between PD and substance use at ages 12–22. Shared environmental factors contributed to early PD and heavier substance use for females. Biological father absence was associated with early PD for boys but not girls, and did not account for the relationship between PD and substance use. The association between early PD and heavier substance use was partially due to between-family confounds, although early PD appeared to qualitatively alter long-term trajectories for some substances (nicotine), but not others (alcohol). Mediation by peer and parental factors did not explain this relationship within families. However, higher peer substance use and lower parental monitoring were themselves associated with heavier substance use, strengthening the existing evidence for these factors as targets for prevention/intervention efforts. Early maturation was not supported as a robust determinant of alcohol use trajectories in adolescence and young adulthood, but may require longer term follow-up. Subtle effects of early PD on nicotine and illicit drug use trajectories throughout adolescence and adulthood merit further investigation.

We offer a brief sketch of an overlooked early twin researcher, Arvo Johannes Lehtovaara (1905–1985), Professor of Psychology at the University of Jyväskylä, 1939–1952, and the University of Helsinki, 1952–1970, with background notes on his mentor, Eino Kaila.

Family, twin and adoption studies have provided evidence for familial and genetic influences on individual differences in disease risk and in human behavior. Attempts to identify individual genes accounting for these differences have not been outstandingly successful to date, and at best, known genes account for only a fraction of the familiality of most traits or diseases. More detailed knowledge of the dynamics of gene action and of specific environmental conditions are needed. Twin and twin-family studies with multiple measurements of risk factors and morbidity over time can permit a much more detailed assessment of the developmental dynamics of disease risk and the unfolding of behavioral risk factors.

We compared twins to their gender-matched singleton classmates in peer-assessed behavioral adjustment. Our samples include 1874 11- to 12-year-old Finnish twins (687 monozygotic, MZ; 610 same-sex dizygotic, SSDZ; 577 opposite-sex dizygotic, OSDZ) and their 23,200 non-twin classmates. Data were collected using a 30-item Multidimensional Peer Nomination Inventory containing three factors and their subscales. We found twin-singleton differences: classmates rated twin girls and boys higher than gender-matched singletons in Adaptive Behaviors (constructive, compliant, and socially active behavior), and those effects were particularly evident among OSDZ twins for assessments of social interaction, popularity, and leadership. We found no evidence that individual twins differ from singletons in Externalizing (hyperactivity- impulsivity, inattention, aggression) or Internalizing Problem Behaviors (depressive symptoms, social anxiety). Nor did we find systematic differences between MZ and SSDZ twins. Among both twins and singletons, boys exceeded girls in Externalizing, and girls exceeded boys in Internalizing Problem Behaviors. Results suggest that a twinship forms a positive developmental environment for socioemotional behavior, particularly among OSDZ twins.

In contrast to many phenotypes that have been studied using twin designs, substance use shows considerable evidence of environmental influence. Accordingly, specifying the relevant environments and understanding the nature of their effects is an important research priority. Twin studies also have demonstrated that the importance of genetic and environmental influences varies across development for a variety of behavioral outcomes, including substance use. Here, we report analyses exploring moderating effects associated with parenting and peer characteristics on adolescent smoking and drinking, measured at ages 14 and 17. We find significant evidence of moderating effects associated with two dimensions of parenting (parental monitoring and time spent in activities with parents) on adolescent smoking, measured at two time points across development, but no moderating effects on adolescent drinking. Genetic influences on smoking increased, and common environmental effects decreased, as adolescents reported less parental monitoring and spending more time with their parents. Conversely, we find evidence that adolescent drinking is more strongly influenced by peer characteristics. The importance of genetic predispositions was increased among adolescents who reported more friends who used alcohol. These analyses illustrate the importance of incorporating measured aspects of the environment into genetically informative twin models to begin to understand how specific environments are related to various outcomes. Furthermore, they illustrate the importance of using a developmental perspective to understand how specific influences may vary across different ages, and across different phenotypes.

We examined whether externalizing problem behaviors (hyperactivity–impulsivity, aggressiveness, and inattention) predict illicit drug use independently, or whether their associations with drug use are mediated through cigarette smoking. We used a prospective longitudinal design within the FinnTwin12-17 study among Finnish adolescents with baseline at age 12 and follow-up surveys at ages 14 and 17. Path models were conducted with Mplus and included 1992 boys and 2123 girls. The outcome was self-reported ever use of cannabis or other illicit drugs at age 17. The predictors were: externalizing behaviors (hyperactivity–impulsivity, aggressiveness, and inattention) assessed by teachers and parents (age 12) and self-reported cigarette smoking (age 14). The findings differed across behavior studied. The association of hyperactivity–impulsivity with drug use was mostly mediated through earlier cigarette smoking. Concerning aggressiveness and inattention, the results were different among girls than boys. Among girls no significant mediation occurred, whereas among boys more consistent evidence on mediation was seen. Consistently in all models, the direct association of early cigarette smoking on drug use was strong and highly significant. We conclude that the associations of externalizing problem behaviors with illicit drug use are partially mediated through cigarette smoking. Although interventions targeting externalizing problem behaviors may protect adolescents from early onset smoking and subsequently experimenting with drugs, interventions to prevent cigarette smoking initiation are also important in reducing risk of later drug use.

A multidimensional peer nomination inventory (MPNI; 30 items) and parallel versions – MPNI-Teacher and MPNI-Parental Rating Forms (37 items each) – were developed during a major new Finnish study of families with twins. The twins (477 girls and 467 boys) were 12 years old, representing subsets of three nationwide Finnish twin cohorts (b. 1983–1985). They were enrolled in 503 school classes, and the total number of children participating in peer nominations was 12 937. Three main factors were extracted from peer nominations and teacher and parental assessments. Intercorrelating sub-components were found, especially in parental assessments. Scales were formed, accordingly, for Behavioral Problems (including Hyperactivity-Impulsivity, Aggression, and Inattention), Emotional Problems (including Depression and Social Anxiety), and Adjustment (including Constructiveness, Compliance, and Social Activity). A framework for the development of the multidimensional inventory was a model of emotional and behavioural regulation. Peer nominations were most reliable, while parental assessments, although mostly satisfactory, were least reliable. Results provided evidence of concurrent validity of peer-referenced assessment, using teacher assessments as criteria; correlations between assessments of peers and parents were lower. The inventory has discriminative validity. Intra-pair correlations of monozygotic co-twins were higher than correlations of same-sex (SS) and opposite-sex (OS) dizygotic (DZ) co-twins for all scales across all three assessors, and peer nominations of both SS and OSDZ co-twins yielded correlations significantly greater than zero for all scales. All scales, except Depression and Social Anxiety, differentiated boys from girls.

This study examined genetic and environmental contributions to the covariance between aggression and hyperactivity-impulsivity as rated by twins' teachers and parents. Sex-differences in these genetic and environmental contributions and rater bias/sibling interaction effects were of interest as well. Part of an ongoing nation-wide twin-family study of behavioral development and health habits, the sample consisted of 1636 Finnish twin pairs ascertained from five consecutive and complete twin birth cohorts. Data were collected at ages 11–12, using teacher and parental rating forms of the Multidimensional Peer Nomination Inventory. Bivariate analyses were performed using structural equation modeling allowing sex-limitation effects. Results show that, in addition to significant genetic and environmental influences specific to each behavior, aggression and hyperactivity-impulsivity share common genetic and environmental etiology. Results provide evidence that both genetic and environmental factors are important in creating the observed correlation between aggression and hyper-activity-impulsivity.

A trend of increasing victimization of women and children in situations of war and community violence has emerged in the 20th century. Children are also exposed to violence in their own families, as witnesses of parental violence, targets of physical abuse, and recipients of TV violence. As regards the direct physical effects of violence, children's injuries are no different from those of adults, while indirect effects comprise increases in, for instance, malnutrition. Psychological effects of danger include post-traumatic stress disorder and maladaptations, such as aggression, cognitive impairment, and truncated moral development. Different scenes of violence are interrelated. The cycle of violence lives its own life, but breaking up the cycle is not a hopeless task.

The participants (originally 196 boys and 173 girls) in an ongoing longitudinal study were examined using peer nomination and teacher rating at ages 8 and 14 years. Criminal records were collected at age 27 years. The results showed that (a) criminal offenses were best predicted if the accumulation of behavior problems over the school years was considered; (b) the risk for different types of offenses was highest for boys who exhibited escalating conduct problems and school failure over the school years; (c) norm-breaking behavior in early adolescence was strongly related to a propensity to later criminal offenses; (d) childhood aggressiveness did not predict arrests without the presence of other problems. When followed by norm-breaking in early adolescence, it predicted violent offenses, and when followed by poor school success, it predicted property offenses; (e) interestingly, low prosociality turned out to have a significant independent relationship to arrests; and (f) the distinction between an early-onset path and a late-onset path proved to be valid.