Wednesday, Dec 07, 2011

Parents heavily influence their kids in many ways; they see it with their own eyes. Their mistake is to assume that their influence lasts a lifetime, instead of fading out as kids grow up... Twin and adoption studies find that the long-run effects of parenting are shockingly small. As long as you don't do anything crazy, your kids will probably turn out fine... Today's typical parents artificially inflate the price of kids, needlessly worry, and neglect the long-run benefits of larger families.

Once upon a time, nature versus nurture was a matter of opinion, but now there are hard answers: Nature wins, especially in the long run. If your child had grown up in a very different family... (s)he probably would have turned out about the same... The secret to unravelling the nature-nurture mystery is to study special kinds of families where there is a clear seam-- or even a complete separation-- between biology and parenting. From this standpoint, two kinds of families are special: Families that adopt, and families with twins... Twin and adoption studies do not support the middle-of-the-road answer. Identical twins are much more similar than fraternal twins-- even when separated at birth-- and their similarity often increases as they age.

Parent don't affect life expectancy. Major twin studies find no influence of family environment on life span. One looked at almost 3,000 pairs of Danish twins born between 1870 and 1900... They found moderate genetic effects... [and] no evidence for an impact of a shared (family) environment. Another study looked at the mortality of about 9,000 Swedish twins born between 1886 and 1925... The Swedish study found strong genetic effects... and zero effect of upbringing.

Parents have little or no effect on overall health... A study of over 3,000 elderly Danish twins found moderate effects of heredity on hospitalizations and self-reported health, but no effects of family environment. Another team of researchers looked at about 2,500 Swedish twins and found moderate genetic effects on self-rated health, but small or nonexistent nurture effects.

Parents don't affect height, weight, or teeth... Genes strongly influence both height and weight, while upbringing influences neither. In the Swedish Adoption/Twin Study of Aging, twins raised together were about as similar in height and weight as twins raised apart, and identical twins were about twice as similar as fraternal twins-- both strong signs that nature matters and nurture doesn't... A major study of adult Swedish twins looked at gingivitis, periodontal disease, and complete tooth loss. All three conditions showed moderate genetic influence. Family environment, in contrast, mattered only for the rare problem of losing all your teeth.

Parents might have a small effect on smoking, drinking, and drug problems...The answers from twin and research aren't completely one-sided. Some conclude that nature fully explains why smoking, drinking, and drug use run in families... But other researchers find that nurture plays a role as well.

A large scientific literature finds that parents have little or no long-run effects on their children's intelligence. Separated twin studies, regular twin studies, and adoption studies all point in the same direction. The Minnesota Study of Twins Reared Apart reunited almost 100 separated identical twins and triplets and gave them two standard IQ tests. It found large effects of genes on adult intelligence: "... Growing up in the same family does not contribute to similarity in cognitive abilities later in life." ... In 1975, the Colorado Adoption Project began studying 245 adopted babies... It also set up a control group of 245 comparable babies being raised by their biological parents. By the age of twelve, adoptees raised in high-IQ homes were no smarter than those raised in average homes.

Parents grossly overestimate their influence [on child happiness]. The Minnesota Twin Registry gave personality tests to over 1,300 pairs of adult twins raised together... Identical twins were far more similar in their happiness than fraternal twins... The same goes for self-esteem. The single most impressive study interviewed almost 8,000 twins from the Virginia Twin Registry and found zero effect of parenting on self-esteem for both men and women.

What about unhappiness? We're quicker to blame our parents for our misery than thank them for our joy, but further research using the Minnesota Twin Registry concluded that nurture is equally irrelevant... The twins' personality test measured many different ways to feel bad: nervous, upset, guilty, mistreated, betrayed, angry, vindictive, and so on. Upbringing didn't matter. By the time you're an adult, your parents' past mistakes are not the reason for your present unhappiness.

Parents have little effect on how much school their kids get... The Korean adoption study is most remarkable for what it failed to find. Rich parents routinely try to give their kids an edge by moving to good school districts, hiring tutors, and paying tuition for fancy schools. Yet neither family income nor neighbourhood income increased adoptees' academic success... Another major study of over 2,000 Swedish adoptees plus their adoptive and birth parents got almost the same results.

Parents have no effect on grades... An early study of about 500 Australian twins reported little or no effect of upbringing on college-bound students' knowledge of arts, science, English, geography, math, or biology. A research team investigating the attitudes of almost 700 Canadian twins discovered that family had no effect on high school seniors' GPA.

Parents have little or no effect on how much money their kids make when they grow up... In Sacerdote's Korean adoption study, biological children from richer families grew up to have much higher incomes, but adoptees raised in the same families did not. The results are strong to the point of shocking. The income of the family you grew up with has literally no effect on your financial success... Identical twins' incomes are much more similar than fraternal twins'. A recent working paper looks at over 5,000 men from the Swedish Twin Registry born between 1926 and 1958. Identical twins turn out to be almost exactly twice as similar in labor incomes as fraternal twins... A study of over 2,000 Australian twins finds the same thing.

Parents have little or no effect on conscientiousness or agreeableness... To answer, we need plausible ways to measure character... Personality psychologists have been there and done that. Some find small effects of family environment on character; the rest find none at all... One team looked at almost 2,000 German twins... they found unusually large effects of nature on conscientiousness and agreeableness, and no effect of nurture... In 2005, leading psychologist John Loehlin took a comprehensive look at family studies of personality: Character does run in families... In 1984, Science published a study of almost 15,000 Danish adoptees age fifteen or older...As long as the adoptee's biological parents were law abiding, their adoptive parents made little difference... If the adoptee's biological parents were criminal, however, upbringing mattered... In 2002, a study of antisocial behaviour in almost 7,000 Virginian twins born since 1918 found a small nurture effect for adult males and no nurture effect for adult females... For outright criminality, however, heredity was the sole cause of family resemblance.

Parents have a big effect on religious labels, but little on religious attitudes and behaviour... One early study of almost 2,000 adult Minnesota twins reared together and apart found little or no effect of parenting on religiosity... Another team of researchers looked at the religiosity of over 11,000 adult twins from Virginia. Parents have almost no effect on adult church attendance.

Parents have a big effect on political labels, but little on political attitudes and behaviour... Twin studies confirm that politics is a lot like religion. Parents have a large effect on your political label... In politics as in religion, however, the biggest nurture effects are also the most superficial. The Virginia 30,000 study found that parents have little effect on the strength of your partisan commitment. A national survey of young American adult twins found that parents have little influence over whether people bother to vote or participate in other political activities... The same goes for overall political philosophy and positions on specific issues.

Parents have little effect on traditionalism and modernism... Diverse twin studies find little or no effect of nurture on openness, including studies of over 1,000 Swedes raised apart and together, almost 2,000 Germans, and about 1,600 American high school juniors, and 500 Canadians.

Parents have moderate influence over when their daughters start having sex, but little over their sons. There are two major Australian twin studies of sexual initiation. The first included over 3,000 women born between 1922 and 1965. A follow-up roughly doubled the sample size by adding older and younger female twins. Both studies found moderate to large nurture effects [for women].

Parents have little or no effect on teen pregnancy... Girls' parents are more likely to take extreme measure due to fear of teen pregnancy. Their precautions largely fail... A study of about 2,000 female Swedish twins born in the Fifties found zero effect of upbringing on teen pregnancy.

Parents have little or no effect on adult sexual behaviour... Two major twin studies find little effect of upbringing on adult sexuality. The first surveyed nearly 5,000 Australian twins about their... promiscuity... Family environment had almost no effect on sociosexuality; if you were in the 80th percentile, you could expect your adopted sibling to stand in the 51st.

Parents have little or no effect on marriage, marital satisfaction, or divorce... A study of over 4,000 Minnesota twins, most in their thirties and forties, found zero effect of parenting on marital status... A research team asked 1,000 female Swedish twins and their spouses about the quality of their marriages. The women's parents had no effect on the marital satisfaction of their daughters, but they did have a small effect on the marital satisfaction of their son-in-law... One of the main reasons why divorce is heritable... is that marital stability depends upon personality and values, which in turn depend on genes.

Both twin and adoption studies confirm that parents affect how their children perceive and remember them... Half a century from now, your children will remember how you treated them... "good memories" are one of the few that clearly depend upon how you raise your child.

Children under five years old are almost five times as safe today as they were in the Idyllic Fifties. Children age five to fourteen are almost four times as safe.

Despite popular fears of overpopulation, more people make the world a better place. Our population and our standard of living have risen side by side for centuries... New ideas... are the main reason we keep getting richer. The source of new ideas, without a doubt, is people.

The critics of genetic determinism aren't just wrong; they're not even listening. Twin and adoption studies never claim that genes fully explain variation in human behaviour... If family environment matters so little, why is human behaviour hard to predict? Because there's a lot more to "the environment" than the family.

The most important weakness of behavioural genetics is simply that research focuses on middle class families in First World countries... So you shouldn't conclude that Haitian orphans would turn out the same way if raised in Sweden... Adoptees almost never grow up in lower-class homes, even though their biological parents tend to be lower class.

Do not be alarmed when twin and adoption studies conclude that your children's future is outside your control. They're not saying that your children will do poorly. They're saying that your children will probably turn out fine, whether or not you're a great parent.

The most effective way to get the kind of kids you want is to pick a spouse who has the traits you want your kids to have.

Selfish Reasons to Have More Kids: Why Being a Great Parent is Less Work and More Fun Than You Think, Bryan Caplan, 2011,

Contradictory evidence:

In the first functional MRI brain scan study to investigate the impact of physical abuse and domestic violence on children, scientists at UCL in collaboration with the Anna Freud Centre, found that exposure to family violence was associated with increased brain activity in two specific brain areas (the anterior insula and the amygdala) when children viewed pictures of angry faces.

Previous fMRI studies that scanned the brains of soldiers exposed to violent combat situations have shown the same pattern of heightened activation in these two areas of the brain, which are associated with threat detection. The authors suggest that both maltreated children and soldiers may have adapted to be ‘hyper-aware’ of danger in their environment.

However, the anterior insula and amygdala are also areas of the brain implicated in anxiety disorders. Neural adaptation in these regions may help explain why children exposed to family violence are at greater risk of developing anxiety problems later in life.

"Not every child exposed to family violence will go on to develop a mental health problem; many bounce back and lead successful lives. We want to know much more about those mechanisms that help some children become resilient.”

Maltreated children show same pattern of brain activity as combat soldiers, University College London, December 5, 2011,