160 VOLUME 9, NUMBER 5, OCTOBER 2000 Three Laws of Behavior Genetics and What They Mean Eric Turkheimer' Department of Psychology, University of Virginia, Charlottesville, Virginia Abstract Behavior genetics has dem- onstrated that genetic variance is an important component of variation for all behavioral out- comes, but variation among families is not. These results have led some critics of behav- ior genetics to conclude that heritability is so ubiquitous as to have few consequences for scientific understanding of de- velopment, while some be- havior genetic partisans have concluded that family environ- ment is not an important cause of developmental outcomes. Both views are incorrect. Geno- type is in fact a more system- atic source of variability than environment, but for reasons that are methodological rather than substantive. Development is fundamentally nonlinear, interactive, and difficult to con- trol experimentally. Twin stud- ies offer a useful methodologi- cal shortcut, but do not show that genes are more fundamen- tal than environments. Keywords genes; environment; develop- ment; behavior genetics The nature-nurture debate is over. The bottom line is that every- thing is heritable, an outcome that has taken all sides of the nature- nurture debate by surprise. Irving Gottesman and I have suggested that the universal influence of genes on behavior be enshrined as the first law of behavior genetics (Turkheimer & Gottesman, 1991), and at the risk of naming laws that I can take no credit for discovering, it is worth stating the nearly unani- mous results of behavior genetics in a more formal manner. First Law. All human behavioral traits are heritable. Second Law. The effect of being raised in the same family is smaller than the effect of genes. Third Law. A substantial portion of the variation in complex hu- man behavioral traits is not ac- counted for by the effects of genes or families. It is not my purpose in this brief article to defend these three laws against the many exceptions that might be claimed. The point is that now that the empirical facts are in and no longer a matter of serious controversy, it is time to turn atten- tion to what the three laws mean, to the implications of the genetics of behavior for an understanding of complex human behavior and its development. VARIANCE AND CAUSATION IN BEHAVIORAL DEVELOPMENT If the first two laws are taken lit- erally, they seem to herald a great victory for the nature side of the old debate: Genes matter, families do not. To understand why such views are at best an oversimplifica- tion of a complex reality, it is nec- essary to consider the newest wave of opposition that behavior genet- ics has generated. These new crit- ics, whose most articulate spokes- man is Gilbert Gottlieb (1991, 1992, 1995), claim that the goal of devel- opmental psychology is to specify the actual developmental processes that lead to complex outcomes. In lower animals, whose breeding and environment can be brought under the control of the scientist, it is possible to document such devel- opmental processes in exquisite de- tail. The critics draw an unfavor able comparison between these detailed animal studies and twin studies of behavior genetics, which produce only statistical conclu- sions about the relative importance of genes and environment in devel- opment. The greatest virtue of the new challenge is that it abandons the Published by Blackwell Publishers Inc. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 161 implausible environmentalist con- tention that important aspects of behavior will be without genetic influence. Gottlieb (1992) stated, "The present... viewpoint holds that genes are an inextricable com- ponent of any developmental sys- tem, and thus genes are involved in all traits" (p. 147). Unlike earlier critics who deplored the reduction- ism they attributed to behavior genetic theories of behavior, the developmental biologists take be- havior genetics to task for not be- ing mechanistic enough. Once vili- fied as the paragon of determinist accounts of human behavior, be- havior genetics is now chastised for offering vague and inconclusive models of development (Gottlieb, 1995; Turkheimer, Goldsmith, & Gottesman, 1995), and judged by the standards of developmental psychobiology in lower animals, it is true enough that behavior ge- netic theories of complex human behavior seem woefully poorly specified. But ultimately the charge is unfair, because there is no equivalent in developmental psy- chobiology to the behavior genetic study of marital status or school performance. The great preponder- ance of the exquisite experimental science that goes into animal psy- chobiology is quite simply impos- sible to conduct in humans. Human developmental social science is difficult equally so for the genetically and environmen- tally inclined-because of the (methodologically vexing, human- istically pleasing) confluence of two conditions: (a) Behavior emerges out of complex, nonlinear developmental processes, and (b) ethical considerations prevent us from bringing most human de- velopmental processes under effec- tive experimental control. Figure 1 is a schematic illustration of the problem. Individual genes (Genes 1, 2, and 3) and their environments (which include other genes) inter- act to initiate a complex develop- mental process that determines adult personality. Most characteris tic of this process is its interactivity: Subsequent environments to which the organism is exposed depend on its earlier states, and each new en- vironment changes the develop- mental trajectory, which affects fu- ture expression of genes, and so forth. Everything is interactive, in the sense that no arrows proceed uninterrupted from cause to effect; any individual gene or environ- mental event produces an effect only by interacting with other genes and environments. For the behavior geneticist, Genes Gene 1 Gene 2 Gme 3 Reinforcement Environment Parenting Variation in penality accounted for by genes Development Personality Human Birth Order "Variation in personality accounted for by environment Fig. 1. Schematic diagram of contrasting roles of genes and environment in development of personality. One-headed arrows link causes to effects; two-headed arrows indicate correlations. Genes and environments are both causal inputs into an interactive developmental system (represented by the network of arrows in the center of the figure), but because people select and shape their own environments (as represented by lighter one-headed arrows from personality to environments), correlations across the developmental system (dotted two-headed arrows) are easier to detect for genes than for environments. Copyright 2000 American Psychological Society however, the quasi-experimental gift of genetically identical and nonidentical twins offers a remark- able, if deceptively simple, method to span this daunting interactive complexity. Thanks to the fact that identical twins are on average ex- actly twice as similar genetically as nonidentical twins, one can use straightforward statistical proce- dures to estimate the proportion of variability in complex outcomes that is associated with causally dis- tant genes, all the while maintain- ing a state of near-perfect igno- rance about the actual causal processes that connect genes to be- havior. This methodological short- cut is not available to rivals of be- havior genetics who seek to measure the effects of families on behavior. How similar was my rearing environment to that of my siblings? And how similar was it to the environment of my adopted sibling, if I have one, or to the en- vironment of my biological sibling who was raised by someone else? The apparent victory of nature over nurture suggested by the first two laws is thus seen to be more methodological than substantive. In a world in which there were oc- VOLUME 9, NUMBER 5, OCTOBER 2000 partitioning sources of variance to specifying concrete developmental processes (Turkheimer, 1998), and although critics like Gottlieb are correct that heritability per se has few implications for a scientific un- derstanding of development, they have failed to emphasize two cru- cial points. First, heritability does have one certain consequence: It is no longer possible to interpret cor- relations among biologically re- lated family members as prima fa- cie evidence of sociocultural causal mechanisms. If the children of de- pressed mothers grow up to be de- pressed themselves, it does not necessarily demonstrate that being raised by a depressed mother is it- self depressing. The children might have grown up equally depressed if they had been adopted and raised by different mothers, under the influence of their biological mother's genes. For every behavior geneticist who continues to report moderate heritabilities as though they were news, there is an envi- ronmentalist who reports causally ambiguous correlations between genetically related parents and children. Second, the problem the critics have uncovered extends have been accounted for, around 50% of the differences among sib- lings is left unexplained. In recent years, scientists interested in the genetics of behavior have come to call this unexplained portion the "nonshared environment." Al- though according to the second law shared environment accounts for a small proportion of the vari- ability in behavioral outcomes, ac- cording to the third law, nonshared environment usually accounts for a substantial portion. So perhaps the appropriate conclusion is not so much that the family environment does not matter for development, but rather that the part of the fam- ily environment that is shared by siblings does not matter. What does matter is the individual envi- ronments of children, their peers, and the aspects of their parenting that they do not share. Plomin and Daniels (1987) reviewed evidence of the predominance of nonshared environmental variance and posed a seminal question: Why are chil- dren in the same family so differ- ent? They proposed that siblings are different because nonshared environmental events are more ERIKSONIAN STAGES OF DEVELOPMENT AND THE IMPACT OF CATEGORIES OF TRAUMA (Note: Each stage has developmental milestones that if unattained and issues that if unresolved will impact later stages) STAGE Infancy (Birth to 18 months) BASIC CONFLICT IMPORTANT EVENTS Trust vs. Mistrust Early Autonomy vs. Childhood (2 to Shame and 3 years) Doubt Feeding & Basic Bonding through eye contact and touch Toilet Training GENERAL OUTCOME Children develop a sense of trust when caregivers provide reliability, care, and affection. A lack of such conditions can lead to mistrust, regression, fear of separation and delay in language acquisition. Children need to develop a sense of personal control over physical skills and a sense of independence. Success leads to feeling of autonomy; failure or overly restrictive parenting can result in shame and doubt with potential disturbances in attachment formation. IMPACT OF PARENTAL/CAREGIVER LOSS/IRREGULAR PRESENCE, OR SERIOUS ILLNESS . PSYCHIATRIC/MEDICAL Parental impairment/absence disrupts the delivery of consistent physical and emotional sustenance and can impact bonding and eye contact. Without a good substitute, the person may experience increased difficulty forming and sustaining relationships that support self-care as well as care for others Lack of food and infant care can result in mental and physical growth delays as well as fixation or regression of developmental milestones such as crawling, walking and talking Schizophrenic parents increase the sense of fear and mental confusion becoming the child's perceived normal state. Any irregular parenting can compromise a sense of hope in or distrust of any future caregivers and can be maintained even into adult relationships. Absent parents or parents with serious mental illness increase the predisposition to more serious forms of depression. toddler of Irregular presence of caregivers deprives th needed structure to proceed with the mastery of toilet training Difficulty with toilet rating can be related to difficulties in developing organization, boundaries, self-control and cleanliness. Overly strict parents increase the likelihood of preoccupations can could potentially lead to obsessive and compulsive traits and anxiety IMPACT OF DOMESTIC VIOLENCE OR SEXUAL ABUSE (never occurs outside the context of abandonment and deprivation) . . Violence and abuse in the home is experienced by the infant as an unreliable and dangerous environment which can increase the child's feelings of irritability and crying which can lead to withdrawal and fear-based behavior. Future issues can include increased likelihood of chaotic and disorganized attachment patterns. Infants may experience difficulty trusting people or environments to be able to provide their needs. If the child experience physical injury/neurological injury a potential impact could be compromised normal physical and neuropsychological growth negatively impacting normal development. People who develop panic disorder sometimes show a relationship to having experienced childhood sexual abuse. A childhood in which violence and abuse is present increases the risk of the child for the following: Potential for perceiving the world as unpredictable and dangerous o Increased potential of an enhanced startle ENVIRONMENTAL SOCIO-POLITICAL TRAUMA (lack of resources and disruption of physical and emotional care). . Decreased availability of resources such as housing, money, food or transportation interferes with adequate physical sustenance for the infant and increases the anxiety of the caregivers. Disruption of meeting the infants emotional needs will diminish appropriate bonding occurring. Characteristics in previous stage may occur along with and increased potential for separation anxiety and the startle response. Increased potential for somatic complaints 0 response Potential of experiencing insomnia, 0 Possible increase in irritability . Potential of developing a specific fear of the caregivers milestones can be delayed. Attainment of developmental Preschool (3 to 5 years) Initiative vs. Guilt School Age (6 to 11) Industry vs. Inferiority If the child experiences extensive shame there is an increase in the vulnerability to developing issues such as Separation Anxiety Disorder and Selective Mutism. Mothers with eating disorders increase the risk of the development of Eating Disorders in the teen years and young adulthood. . . 0 Increased potential for clingy behavior. Over time, the person is prone to increased potential for fear and paranoia Higher probability of experiencing regression to earlier stages of development in adulthood May experience love/relationships as dangerous. Child likely to fear caregivers and demonstrate temper outbursts. May acquire chaotic and disorganized attachment patterns in adolescence and adulthood. The child may experience increased confusion about love or may associate love with violence in future relationships. Exploration Children need to begin asserting control and power over the environment. Success in this stage leads to a sense of purpose. Children who try to exert too much power experience disapproval, resulting in a sense of guilt. The development of autobiographical memory may be affected along with a decreased ability to cope with stress and the possible initiation of permanent changes in neurobiology. Sexual trauma may potentially make toilet training and normal sexual exploration may be interrupted leading to potential marked confusion. Guilt-laden responsibility for the condition of the parents Autobiographical memory may be compromised. Tendency toward depression and suicide increases if a parent dies. . Potential for PTSD symptoms with nightmares and intrusive recollections specific to the trauma The child's attempts to exert control over the environment . . Potential for identification with the perpetrator and may identify with being a victim with a possible inclination. to form relationships with the potential of abuse. Potential impact resulting in learning difficulties, acting out, difficulty making friends, bedwetting and poor self-esteem. are thwarted by forced relocation and/or the reduction of resources. . Identification with parent's anxiety, rage and/or Confusion, denial through over activity and compromised self-esteem may be increased. The child is bereaved and needs an alternative caregiver to help them grieve. Older siblings tend to be relied on as caregivers which interferes with them recovering or grieving. This may compromise their own achievement of age-appropriate developmental tasks. Will tend to retreat from caregivers, showing a tendency towards passivity, withdrawal and the disruption of normal separation and individuation Separation of siblings by placement in different homes constitutes another loss. This can worsen the psychological symptoms such as: Hopelessness, anger, demoralization and feeling of abandonment, as well as guilt for feeling these emotions. This will tend to prolong the recovery process. Child's reactions commonly interferes with normal play. . Increased risk of developing psychological disorders, substance abuse, criminal behavior and sexual risk-taking. . Increased risk of difficulty in the intimacy vs isolation stage. Parental overprotection can increase the risk of developing agoraphobia, generalized anxiety disorder, Exhibitionistic Disorder and Pedophilia. Formation of healthy conscience can be effected regarding what constitutes appropriate actions, empathy, appropriate guilt and self-control. Because of these types of reaction there is an increased risk of Intermittent Explosive Disorder, Conduct Disorder, and/or bulimia nervosa. confusion about what is occurring is more likely Typically, children require stable parents who give age appropriate explanations about changes in the environment. Multiple and varied adverse experiences can strongly increase the potential risk for the development of Major Depressive Disorder. Extensive changes in ambient light, noise and environmental disruption can lead towards the development of Insomnia Disorder. Children can experience increased confusion, denial through over activity and compromised self-esteem. The child experiences bereavement which is best addressed by having an alternative caregiver to help them grieve. . Characteristic effects of earlier ages may be present . Displacement from school with stronger tendencies towards poor emotional control and extreme vacillations in behavior. interrupts the healthy development of normal social . relationships with peers and School Children need to cope with new social and academic demands. Success leads to a sense of competence, while failure leads to role confusion and a weak sense of self. Older siblings tend to be relied on as caregivers which prevents them from effectively recovering or grieving which can compromise the child achieving age-appropriate developmental tasks. Separation of siblings by placement in different homes constitutes another loss which can worsen the psychological symptoms. Feeling of accomplishment with school success can be reduced as being perceived as a part of ego-building, unless a nurturing caregiver is stably present. A nurturing teacher helps. Poor school performance is more likely Progression through the grades can be negatively impacted. Acting out behavior, development of bullying, withdrawal and school refusal are more likely. . Children tend to have increased expectation that others are dangerous and are unable to judge if a threat is serious The child feeling the need to protect parents can increase the potential for school phobia, acting out at school, learning disabilities, and overprotective behavior regarding the parents. alternative authority figures. Concern for personal safety and the emotional condition of parents may result in decreased school performance as well as somatic complaints. Increased risk of developing Illness Anxiety Disorder. Trauma and Development: Basic Data 1. Traumatic Memories have the following qualities: a. Disorganized b. May not have consistent sequence of events c. Flashbacks d. May be triggered by scent, visuals, sounds, taste, or other sensory input 2. What is traumatic to young children? Keep in mind that children are concrete thinkers. a. Physical abuse b. Sexual abuse c. Emotional abuse d. Neglect (physical or emotional) e. Domestic violence in the home f. Community violence (hearing about abuse) g. Natural disasters (to include families having "intruder drills") h. Primary caregiver mental illness or addiction i. Losing a home (eviction, fire)/homelessness/highly transient families j. Media violence (TV, movies, video and computer games) k. Death of primary caregiver or family member (don't protect children from death; don't give easy explanations that avoid the reality; but do use age-appropriate ideas for helping them understand and cope) 1. Severe illness (self) m. Severe illness (primary caregiver) n. Frequent hospitalization / medical procedures o. Foster-care placements p. Disrupted placements (including military deployment and adoption) 3. Biological Trauma that can cause behavioral symptoms a. History of head injury b. In utero drug exposure c. Prenatal issues and care (high risk pregnancy) d. GI issues e. Compromised vision (darkened rooms may be a trigger) f. Hearing impairment g. Speech/language delay h. Seizure disorder i. Sleep deprivation i. Impacts short-term memory (hard to move these to long-term memory) ii. Reduced frustration tolerance iii. Mood swings iv. Meltdowns v. Poor judgment vi. Impacts depth perception vii. Impacts decision-making 4. Trauma and the Brain a. Babies are born with all the neurons they will ever have; approximately 100 billion, but few synapses b. Synapses develop in response to stimulation (learning) c. These synapses form networks in the brain: "Neurons that fire together, wire together." d. Unstimulated or under stimulated neurons are "pruned off"" e. Traumatic events trigger chemical changes in the brain, including increases in stress hormones such as cortisol and adrenaline resulting in hyperarousal. f. The brain responds by activating the fight or flight response, including hypervigilance to threats. On-going trauma causes this to be an on-going reaction, with no relief. g. The longer children are exposed to trauma, the more affected their brains can be h. PTSD causes stress hormones to remain at high alert, never reach baseline levels i. Response to trauma is mediated in the brain by the locus coeruleus: Regulates attention, arousal, sleep, learning, memory, anxiety, pain, startle response, and irritability. j. Repeated or prolonged exposure to trauma / stress can lead to increased sensitivity in the brain to these reactions and circumstances surrounding the trauma. k. Note: You are not to diagnose ADHD until you can rule out trauma and Traumatic Brain Injury (TBI) 1. Kindling: Exposure to trauma/ stress at critical phases in development becomes encoded in the brain and sensitizes the person to react disproportionally when facing triggering experiences / stimuli m. Repeated exposure to trauma leads to overstimulation of the brainstem and midbrain which can result in attention problems, impulsivity, and poor emotional self-regulation. n. (Lester, et all, 2003) o. Violent behaviors in youth "...are not senseless, they are not beyond our understanding... they arise from children reflecting the world in which they have been raised." (Perry, B.D. 1996)