Background
The research on genetics can be divided into three groups.
Attachment theory makes strong claims about the innate and universal bias in human infants to become attached emerging from millions of years of variation and selection of behavioural systems in the Environments of Evolutionary Adaptedness. But the environment, for the infant to a large extent the parents, provides the input for the universal mechanism. Different attachment behaviour patterns are thought to result from the infants’ experiences with different levels of parental sensitivity. Parents are assumed to differ in sensitivity because of their own childhood attachment experiences (‘intergenerational transmission’). This would imply a rather larger role for the (shared) environment rather than for the genes in determining the quality of attachment. In our programme, the idea of the 'transmission gap' was developed: parental and infant attachments are strongly associated but parental sensitive behaviour can only partly account for the mechanism of transmission (Van IJzendoorn, 1995). The genetic component may provide a bridge to close the gap. These hypotheses have been tested in a series of behavioural and molecular genetic studies.
Behavioural genetic studies
A study on attachment in twins was the first step in this line of research (Bokhorst, Bakermans-Kranenburg, et al., 2003; Bakermans-Kranenburg, Van IJzendoorn, Bokhorst, & Schuengel, 2004). In a sample of 157 monozygotic and dizygotic twins, genetic and environmental influences on infant attachment and temperament were quantified. Only unique environmental or error components could explain the variance in disorganized versus organized attachment as assessed in the Ainsworth Strange Situation Procedure. For secure versus nonsecure attachment, 52% of the variance in attachment security was explained by shared environment, and 48% of the variance was explained by unique environmental factors and measurement error. The role of genetic factors in attachment disorganization and attachment security was negligible. Genetic factors explained 77% of the variance in temperamental reactivity, and unique environmental factors and measurement error explained 23%. Differences in temperamental reactivity were not associated with attachment concordance. Similar results were found for attachment security of the infant-father relationship as assessed with the Attachment Q-Sort (Bakermans-Kranenburg, Van IJzendoorn, Bokhorst, & Schuengel, 2004).
In a multivariate behavioural genetic study, we included both maternal sensitivity and infant attachment of the Bokhorst et al. (2003) twin sample, and examined the extent to which genetic and environmental aspects of maternal sensitivity accounted for the pattern of similarity and dissimilarity in twins’ attachments to their mother (Fearon et al., 2006).
Bivariate behavior genetic models use the differences in within-twin and cross-twin correlations to estimate genetic, shared environmental and non-shared environmental correlations between two measures. We found no evidence for a contribution of genetic factors (residing in the infants) to differences in maternal sensitivity. Shared environmental factors explained 66% of the variance in sensitivity as experienced by the child and non-shared environmental factors 34%. Using bivariate behavior genetic analyses it was then tested whether shared environmental and/or non-shared environmental processes were responsible for the link between sensitivity and attachment (since there was no genetic contribution to maternal sensitivity). Shared variance in maternal sensitivity accounted for part of the similarity between twins in attachment security, but weak non-shared associations between sensitivity and attachment suppressed the magnitude of the correlation between attachment and sensitivity in twin children. The attachment security of one twin appears to depend on the relationship the parent has with the other twin. These findings underscore the importance of effects of relationships on relationships within a family system.
Folder Twinpad (pdf)
Folder Twinstap (pdf)
Molecular genetic studies
We also started the application of DNA genotyping in the context of the ongoing twin study and several other attachment studies. First we attempted to replicate an Hungarian study (Lakatos et al., 2000, 2002) reporting an association between infant disorganized attachment and the DRD4 7-repeat allele (Bakermans-Kranenburg & Van IJzendoorn, 2004), a polymorphism that is associated with motivational and reward mechanisms and Attention Deficit Hyperactivity Disorder (ADHD) in children. We did not find this association in our study; and several other studies did not replicate the Hungarian results either (see Bakermans-Kranenburg & Van IJzendoorn, in press). Our growing interest in the interplay between nature and nurture led to several studies on gene-environment interaction (Bakermans-Kranenburg & Van IJzendoorn, 2006; Van IJzendoorn & Bakermans-Kranenburg, 2006). We found that the combination of the DRD4 7-repeat polymorphism and maternal insensitivity predicted significantly more externalizing behaviour in preschoolers. These results point to a gene - environment interaction effect: maternal insensitivity was associated with externalizing (oppositional, aggressive) behaviors, but only in the presence of the DRD4 7-repeat polymorphism. The increase in externalizing behaviors in children with the 7-repeat allele exposed to insensitive care compared to children without these combined risks was sixfold, indicating that children are differentially susceptible to insensitive parenting dependent on the presence of the 7-repeat DRD4 allele. In a similar vein (but in a different sample) for disorganized attachment a moderating role of the DRD4 gene was found: Maternal unresolved loss or trauma was associated with infant disorganization, but only in the presence of the DRD4 7-repeat allele. Children with the short DRD4 allele did not show higher scores for disorganized attachment when their mother was unresolved. The increase in risk for disorganization in children with the 7-repeat allele exposed to maternal unresolved loss/trauma compared to children without these combined risks was 18.8 fold.
Gene-environment interactions interpreted in terms of differential susceptibility may play a large part in the explanation of individual differences in human development. A recent review presents evidence for interactions between genetic and environmental factors explaining individual differences in attachment security and disorganization (Bakermans-Kranenburg & Van IJzendoorn, in press). In particular the DRD4 7-repeat polymorphism seems associated with an increased risk for disorganized attachment, but only when combined with environmental risk. Gene-environment (G x E) interactions may be interpreted as genetic vulnerability or differential susceptibility. We found support for the differential susceptibility hypothesis predicting not only more negative outcomes for susceptible children in unfavorable environments, but also positive outcomes for susceptible children in favorable environments.
The next step in this line of research involved an experimental intervention study examining whether toddlers’ genetic differences may explain differential effectiveness of an intervention aimed at decreasing externalizing behavior through improving parental discipline and interaction skills. This first experimental test of (measured) gene by (observed) environment interaction in human development indicated that children may indeed be differentially susceptible to intervention effects depending on genetic differences (Bakermans-Kranenburg, Van IJzendoorn, Pijlman, Mesman, & Juffer, in press). In a randomized control trial 157 families with 1- to 3-year-old children participated in a study implementing Video-feedback Intervention to promote Positive Parenting and Sensitive Discipline (VIPP-SD), with six 1½-hour intervention sessions focusing on maternal sensitivity and discipline. Toddlers were screened for their relatively high levels of externalizing behavior. A moderating role of the dopamine DRD4 polymorphism was found: VIPP-SD proved to be effective in decreasing externalizing behavior in children with the DRD4 7-repeat allele. VIPP-SD effects were largest in children with the DRD4 7repeat allele whose parents showed the largest increase in the use of positive discipline.
The Focus Cohort Attachment Project
Broader context: The Generation R Study
The Generation R Study examines growth, development and health in a contemporary population-based cohort of 10.000 urban children from fetal life until young adulthood in Rotterdam, the Netherlands (Hofman et al., 2003). With an integrated approach of epidemiological, clinical and basic research it focuses on four primary areas of research:
- growth and physical development
- behavioural and cognitive development
- diseases in childhood
- health and healthcare for pregnant women and children
Prenatal pregnant women are assessed at 12, 20 and 30 weeks of gestation to collect information about fetal growth and its main determinants. Postnatal, the children participate in a prospective birth-cohort study until the age of 20 years.
The Focus Cohort
Additional detailed assessments of fetal and postnatal growth and development are conducted in a subgroup of about 1000 children and their parents. This subgroup is called the Focus Cohort. The aim of this study is to examine etiological associations with methods that cannot be used in the whole cohort. Examples of assessments in the Focus Cohort are: structured psychiatric interviews of both parents; ultrasounds of the brain, heart, aorta and kidney development; biological samples of blood, urine and saliva.
The Focus Cohort Attachment Project
This project adds to the ongoing Focus Cohort research activities the collection at 14 months of Strange Situation data, observations of parental frightening behaviors during the stressful setting of a medical examination, observations of infant stress reactivity, and the determination of parenting and attachment-relevant polymorphisms in DNA derived from blood samples. At 36 months data on children’s emergent morality such as control of negative emotions, behaviour in a ‘do-task’ and a ‘don’t-task’ have been collected, as well as observations of the quality of the parent-child interactions experimental task settings. In a series of studies the issue of differential susceptibility is addressed: are children more or less susceptibility for parenting on basis of their genetic make-up in the development of attachment, emotion regulation, and emergent morality?