Twins, adoptees and heritability: what behavioral genetics can and can't tell us

Transcript

SoheeA client says it flatly, like a weather report. Depression runs in my family. My mom, my grandmother. It's in my genes, so there's really nothing I can do about it.

RyanAnd part of that is true, and part of it is a misunderstanding. Today we sort out which part is which.

SoheeWelcome to EPPP Prep, the podcast. I'm Sohee.

RyanI'm Ryan. This is behavioral genetics. The OpenStax psychology textbook says behavioral geneticists study how individual differences arise, in the present, through the interaction of genes and environment.

SoheeWhich separates it from evolutionary psychology, which looks at universal patterns of behavior that evolved over long periods. That contrast shows up on the exam.

RyanThe main tools are twin studies and adoption studies. Twin studies compare how often a trait is shared by identical versus fraternal twins. Adoption studies compare biological relatives with adoptive relatives.

SoheeQuick genetics refresher. Identical twins develop from a single fertilized egg that split, so their genetic material is the same. Fraternal twins usually come from two eggs and two sperm, so they vary like ordinary siblings. MedlinePlus says fraternal twins share, on average, 50 percent of their DNA.

RyanSo if identical twins are more alike on a trait than fraternal twins raised together, genes probably play an important role. That comparison is how heritability has historically been estimated.

SoheeThe key statistic is concordance: how often, when one twin has a disorder, the other has it too. Let's run the figures from OpenStax. For major depressive disorder: 50 percent in identical twins versus 38 percent in fraternal twins.

RyanBipolar disorder: 67 percent versus 16. The textbook says that suggests genetic factors play a stronger role in bipolar disorder than in major depression.

SoheeAnd family risk tells the same story. Relatives of people with major depression have double the risk. Relatives of people with bipolar disorder have over nine times the risk.

RyanObsessive-compulsive disorder, OCD: about 57 percent concordance in identical twins and 22 in fraternal twins. The textbook calls that a moderate genetic component.

SoheeAttention-deficit/hyperactivity disorder, ADHD: a review of 26 studies found a median concordance of .66 for identical twins and .20 for fraternal twins. Another review put the heritability of inattention at 71 percent and hyperactivity at 73.

RyanAnd there's a subtle data point in that review. Unrelated adoptive siblings, who share a home but not genes, had a median concordance of .09. Small, but above zero, which the textbook reads as a sign that environment has at least some influence.

SoheeAnd autism appears strongly influenced by genetics, with identical twin concordance of 60 to 90 percent versus 5 to 10 percent for fraternal twins and siblings.

RyanSchizophrenia: having a parent with schizophrenia makes the risk nearly six times greater, and the risk rises as genetic relatedness rises.

SoheeNow the limitation, which the exam cares about. The textbook says family and twin studies can't completely rule out shared environments. Closer relatives tend to share more similar environments, and identical twins may be treated more alike than fraternal twins.

RyanAdoption studies help correct that, because children are separated from their biological parents early. One of the first, by Heston in 1966, followed 97 adoptees. Five of the 47 born to mothers with schizophrenia, about 11 percent, later developed schizophrenia. None of the 50 control adoptees did.

SoheeAnd then the study that shows genes and environment working together. Tienari and colleagues, 2004, studied 303 adoptees. 145 had biological mothers with schizophrenia, the high genetic risk group. 158 had mothers with no psychiatric history.

RyanResearchers also rated the adoptive families as healthy or disturbed, with lots of criticism, conflict and poor problem-solving counting as disturbed.

SoheeThe results. High genetic risk plus a disturbed family: 36.8 percent developed schizophrenia or another psychotic disorder. High genetic risk with a healthy family: 5.8 percent. Low genetic risk with a disturbed family: 5.3. Low risk, healthy family: 4.8.

RyanPause and think. What does the Tienari pattern support, and why isn't it just a genetic story?

SoheeIt supports the diathesis-stress model. The high-risk adoptees were much more likely to develop schizophrenia or another psychotic disorder when raised in a disturbed home. In a healthy home, their rate was close to the low-risk groups. Genes alone don't tell the full story.

RyanLet's define diathesis-stress, since it's the frame for this whole topic. A diathesis is a predisposition. People with one are more likely to develop a disorder when they face adverse events, the stress. And the diathesis isn't always biological. It can be psychological, like a pessimistic thinking style.

SoheeThe key assumption is that both are necessary. And the textbook adds a nice detail: the level of stress needed to produce the disorder is inversely proportional to the level of diathesis.

RyanSo a strong predisposition needs less stress to tip over, and a weak one needs more.

SoheeDepression has a famous molecular example. Caspi and colleagues, 2003, looked at a region of the serotonin transporter gene called 5-HTTLPR. People who'd faced several stressful life events were significantly more likely to have major depression if they carried one or two short versions, rather than two long versions.

RyanBut people with short versions were unlikely to have an episode if they'd faced few or no stressful events. Same gene, different outcome depending on life experience.

SoheeAnd a 2013 study in the United Kingdom found childhood maltreatment before age nine raised the risk of chronic adult depression in people with one or two short versions, but not in those with two long versions.

RyanOne honest caution on those serotonin gene findings. A 2018 collaborative meta-analysis in the journal Molecular Psychiatry pooled 31 data sets, more than 38,000 people of European ancestry, and its findings did not support the gene-by-stress interaction. What it did find, consistently, was that stressful life events are a strong risk factor.

SoheeSo know Caspi as the textbook's example of gene-environment interaction, but don't treat that specific gene finding as settled. The authors note it has generated controversy since it was first proposed in 2003.

RyanThe textbook also gives two other ways genes and environment relate. Range of reaction: genes set the boundaries, and environment determines where within that range you land.

SoheeAnd genetic environmental correlation: genes and environment influence each other in both directions. The textbook's example is a child of an NBA player, who probably grows up around basketball. The parent's genes shape the environment, and that environment supports the child's genetic potential.

RyanNow, heritability itself, from MedlinePlus Genetics. It's a statistical estimate of how much of the variation in a trait across people can be attributed to genetic variation. It's specific to one population in one environment, and it can change as circumstances change.

SoheeIt ranges from zero to one. Close to zero means nearly all the variation comes from environment. Close to one means nearly all comes from genetic differences.

RyanAnd MedlinePlus lists the misconceptions, which are pure exam material. First: a heritability of 0.7 doesn't mean a trait is 70 percent caused by genes. It means 70 percent of the variability in that population is due to genetic differences.

SoheeSecond: knowing heritability doesn't tell you which genes or environmental factors are involved.

RyanThird: heritable isn't the same as familial. Language runs in families, but it's not heritable at all.

SoheeAnd fourth, the one for our client. Heritability says nothing about how easy or hard a trait is to change. MedlinePlus's example: hair color is highly heritable, and it's easy to change with dye.

RyanPause and think. A research summary says the heritability of a trait is 0.6. A student concludes that 60 percent of each person's trait comes from their genes. What's wrong?

SoheeHeritability describes variation in a population, not the makeup of one person. It means about 60 percent of the differences among people in that population, in that environment, are linked to genetic differences.

RyanAnd MedlinePlus reminds us that conditions like schizophrenia are complex, or multifactorial. They're influenced by multiple genes combined with lifestyle and environmental factors, and they don't follow a clear-cut pattern of inheritance, even though they often cluster in families.

SoheeSo, the traps. Treating concordance under 100 percent as proof of no genetic role. Forgetting that twin studies can't fully separate shared environment. Reading heritability as a personal percentage. And assuming heritable means unchangeable.

RyanRecap. Twin studies compare identical and fraternal twins. Adoption studies separate genes from rearing environment. Concordance is higher in identical twins for depression, bipolar disorder, OCD, ADHD, autism and schizophrenia, with bipolar more strongly genetic than depression.

SoheeTienari showed genetic risk mattered mostly in disturbed homes, supporting diathesis-stress. Caspi reported that the short serotonin transporter variant raised depression risk mainly after stressful events, though a large 2018 meta-analysis didn't support that interaction.

RyanAnd heritability describes populations, not individuals. It doesn't name genes, it isn't the same as familial, and it says nothing about whether a trait can change.

SoheeSo back to our client. Yes, depression in a family can mean a real genetic predisposition. But a predisposition is not a sentence. Stress matters, environment matters, and treatment matters.

RyanAnd if you've had your own setbacks on this exam, the same logic applies to you. Where you started doesn't decide where you finish. Thanks for listening to EPPP Prep.

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