negatively regardless of their genetic risk; maltreated children were more likely to misbehave than non-maltreated children in every risk group. Predicting how much more likely is where genes came into play. Though an excellent model, the weakness of Jaffee’s study lies in its generality. Much like Meaney’s experiments with Type A and Type B mice, Jaffee showed that genes influence our susceptibility to abuse without targeting any one gene. In order to understand the relationship between genes and environment, we need to narrow our focus to a precise genetic target.
Pleasure, Anger, and Dopamine
Dopamine is, arguably, the most powerful motivator in the human brain’s arsenal. Without it our entire species would collapse into despondency, indifference, and extinction. But dopamine is no good on its own. In order to reap its benefits, our brains need a way to process it. For this, we have a protein product called a dopamine receptor, a tiny neurological trigger fired by contact with a dopamine molecule. The human brain has many different types of these receptors, but perhaps none has come under more scientific scrutiny than the DRD4 family, a class of dopamine receptors implicated in a host of adverse conditions, including schizophrenia, Parkinson’s disease, bipolar disorder, anorexia nervosa, bulimia nervosa, and drug addiction. A protein product, DRD4 is coded for by the DRD4 gene, and much of its reputation as a hotbed of neurological strife comes from a single allelic variation called the 7-repeat allele. As its name suggests, 7-repeat is a variation of the DRD4 gene where a brief sequence of 48 nucleotides (the microscopic molecules from which genes are constructed) repeats seven times.
Repetition is not what makes the 7-repeat unusual; different varieties of DRD4 have the same sequence repeated anywhere from two to eleven times. What makes the 7-repeat allele unique is its weaker-than-average reaction to dopamine, compared with other versions of DRD4. To put it crudely, the 7-repeat allele is not all that good at its job. While its compatriots handle influxes of dopamine with ease, 7-repeat struggles to stay on target. This idiosyncrasy has made 7-repeat a subject of great interest to scientists across multiple disciplines, as a person’s neurological response to dopamine influences, on a very fundamental level, how that person behaves. It seems inevitable that such an infamous allele — particularly one so intimately involved in regulating human behaviour — would catch the eye of perceptive scholars studying child development.
Among those paying attention were Dutch researchers Marian Bakermans-Kranenburg and Marinus Van IJzendoorn. Together, the two professors from Leiden University in the Netherlands have spearheaded multiple studies investigating the effects of 7-repeat on children’s behaviour. They are, arguably, among the world’s foremost experts on the subject. And they have found a link between the troublesome gene, early rearing environments, and children’s behaviour that you, having read the results of Joan Kaufman’s studies earlier this chapter, may find awfully familiar.
First, Bakermans-Kranenburg and Van IJzendoorn recruited 47 mothers when their children were 10 months old and followed them until the children were three. They visited each mother at home and filmed her performing normal, unstructured activities — cleaning, cooking, feeding, or changing or playing with her child, etc. Mothers weren’t told what the two researchers were looking for, or even whether they or their children were the focus of the study. They were instructed to act as normally as possible and, after a brief period of self-consciousness, settled quickly into their typical routine.
Bakermans-Kranenburg and Van IJzendoorn studied the tapes, documenting the behaviour of each mother-child pair. In the children’s case, they were interested in signs of externalizing behaviour — talking back, hitting, throwing food and toys, fits of anger, hyperactivity, any outward display of excessive aggression or unchecked energy. For mothers, the focus was a bit different. Less attention was paid to actions, and more to interactions. Bakermans-Kranenburg and Van IJzendoorn wanted to gauge something called maternal sensitivity, a variable that measures how adept mothers are at anticipating, interpreting, and responding to their children’s needs.
Mothers’ actions were rated on the Maternal Sensitivity Scale, a sociological measure comprised of four categories: a mother’s awareness of her child’s signals of needs or wants, her accurate interpretation of those signals, the appropriateness of her response, and how promptly she responded. Mothers who scored on the upper half of the scale — meaning they responded quickly and appropriately to their children’s needs — were placed in the high-sensitivity group, and mothers on the lower half — those who responded slowly or infrequently to their children, or who misinterpreted their needs — were classified low sensitivity.
To acquire the study’s final variable, researchers took DNA samples from each child participating in the study and determined which polymorphism (or variation) of the DRD4 gene they possessed. More specifically, they wanted to know whether or not the children had the infamous 7-repeat allele. Bakermans-Kranenburg, Van IJzendoorn, and their observation team were kept clear of the results of these tests until after they had reviewed the tapes and submitted their appraisals of mother and child behaviour. Sequestering data is critical in these types of studies, where objectivity is a goal that is exceedingly difficult to reach, and the slightest potential for bias in the researcher can poison the entire experiment. If the researchers charged with scoring mother-infant behaviour knew which children had the volatile 7-repeat allele, they could — either consciously or unconsciously — rank them higher on the externalizing behaviour scale in order to prove their hypothesis correct. This problem — called confirmation bias — is a constant hazard to experiments in the social sciences, and researchers must guard against it vigilantly.
When all the data was in place, Bakermans-Kranenburg and Van IJzendoorn put it through rigorous analyses. The results were significant. For children with the 7-repeat allele, mothers’ behaviour greatly influenced their odds of acting out. The 7-repeat children with low-sensitivity mothers (mothers who misinterpreted their children’s needs, or who responded indifferently to them) exhibited a great deal of externalizing behaviour, while 7-repeat children with high-sensitivity mothers (mothers who understood their needs and met them promptly) barely showed any externalizing behaviour at all. They were less than half as likely as 7-repeat children with low-sensitivity mothers to talk back, act out, or show signs of ADHD. Children without the 7-repeat allele fell somewhere in the middle, though they were closer to the well-behaved 7-repeat high-sensitivity children than the defiant 7-repeat low-sensitivity children. Their mothers’ level of sensitivity didn’t make much difference to their odds of displaying externalizing behaviour.
Bakermans-Kranenburg, M.J., and Van IJzendoorn, M.H. (2006). “Gene–Environment Interaction of the Dopamine D4 Receptor (DRD4) and Observed Maternal Insensitivity Predicting Externalizing Behavior in Preschoolers.” Developmental Psychobiology, 48(5), 406–409.
Keep in mind that the women in the low-sensitivity group were not necessarily bad mothers. The study did not recruit parents accused of child abuse or neglect, nor did it focus on families from groups generally considered to be high-risk (impoverished, uneducated, suffering from alcoholism or drug addiction, prone to violence, etc.). Low-sensitivity mothers didn’t leave their children home alone for hours on end to go party, or drink themselves sick, or bully their children with taunts or smacks or insults. Some of them were absent-minded. Many of them were overworked and exhausted and didn’t have the energy or patience required to meet their children’s every immediate need, but loved their children all the same and wanted nothing but the best for them. The vast majority were probably doing their best, and would be horrified at the thought that their actions might be hurting their children. And yet, their children were over twice as likely to display aggressive or violent behaviour as those of high-sensitivity mothers — assuming, of course, the children in question had the 7-repeat allele of the DRD4 gene. That is not a small discrepancy, and it shows that in parenting, the little things matter just as much as the big ones.
Fixing the Problem
Having established a link between genes, maternal sensitivity, and children’s behaviour, Bakermans-Kranenburg and Van IJzendoorn strove to use this knowledge to help those children who were most at risk. Their study showed that children with the 7-repeat allele and low-sensitivity mothers were by far the most likely to display