How Genetics and Evolution Shape Psychological Vulnerability

UPI Study › Courses › Abnormal Psychology › Free lessons › Lesson 3.4 Free with a free account

How Genetics and Evolution Shape Psychological Vulnerability

Abnormal Psychology · Chapter 3: Biological Underpinnings of Psychological DisordersSection 3.4 · lesson 12 of 9718 min · free with a free account

← The Brain's Ability to Rewire: Plasticity and AdaptationNext lesson: Medically-Based Interventions, and Weighing the Biological Approach →

Earn 3 college credits for Abnormal PsychologyRecommended for credit by NCCRS · Psychology 180 · self-paced · one payment, lifetime access
Enroll in Abnormal Psychology · $250 →Download the syllabus (PDF)

In this lesson

What each design can establish, what heritability actually means, and what it never means.

  • Explain how genetic influence is studied, and what each design establishes
  • Interpret heritability correctly, and identify the common misreadings
  • Separate gene-environment interaction from gene-environment correlation
  • State the evolutionary account, and its testability problem

Transcript

Everything said in the video, section by section (2,461 words). Read along, or search it.

Biological Underpinnings of Psychological Disorders

How Genetics and Evolution Shape Vulnerability

Conditions run in families. Everybody knows that, and almost everybody misreads it. This section is about what the evidence actually establishes. How genetic influence is studied, design by design. What each one buys you, and what each one cannot separate. Then heritability, which is the single most misquoted statistic in psychology. We define it precisely, and then correct six common misreadings of it. Then two relationships between genes and environments that get confused constantly. Interaction, and correlation. They are not the same thing. And finally, why vulnerabilities persisted at all, if they reduce functioning. That last account is interesting, and it has a testability problem we will name.

Before You Begin

Two Statements That Govern Everything Here

Two notes before we begin, and then two statements that govern the whole section. This material is educational. It is not a basis for assessing family risk. Nor for any reproductive or medical decision. Genetic counseling is a specialist role, and real questions belong with one. Now the two governing statements. Heritability is a statistic about variation within a population. It is not a statement about any individual person. It never tells you what proportion of someone's condition was genetic. And heritability within a population says nothing about differences between populations. Genetic material has historically been used to support fatalism and hereditary blame. And eugenic reasoning, and claims about group differences. None of those is supported by anything here. Each is a misuse of the evidence.

Vocabulary

The Distinction That Does the Most Work

A gene is a stretch of DNA carrying instructions for building a protein. Or for regulating when other genes are active. Genes are arranged along chromosomes, the thread-like structures inside almost every cell. Humans typically have twenty-three pairs. One member of each pair is inherited from each parent. Most genes come in several versions, and a version is called an allele. Because chromosomes come in pairs, a person carries two alleles of most genes. Now the distinction that does more work than any other idea here. The genotype is the person's genetic make-up. The phenotype is the observable characteristic that results. And the phenotype depends on the genotype, on the environment, and on their interaction. Two people with identical genotypes can have different phenotypes. And two people with different genotypes can have the same phenotype. So any sentence that slides between the two has already gone wrong. She has the gene for it is that sentence.

Polygenic

There Is No Gene for Any Condition Here

For a small number of physical conditions, a single gene determines the outcome. Inherit the relevant variant, and the condition follows. It is natural to expect psychological conditions to work the same way. Decades of searching established that they do not. Psychological conditions are polygenic. Influenced by very many genes, each contributing a small amount. The number of contributing variants is large. Current estimates for common conditions run to hundreds or thousands of variants. Each individual effect is very small. No single common variant raises risk by a clinically meaningful amount on its own. Knowing a person's status at any one of them tells you almost nothing. And the same variants are implicated across different conditions. Genetic influence does not respect diagnostic boundaries. Which connects directly to the comorbidity problem from chapter one. If the same variants contribute to several categories, the categories may be cut wrongly. So here is a phrase to retire. There is no gene for depression, for anxiety, or for any condition in this course. Reports of such a discovery concern one small-effect variant among thousands. Or they have been overstated in the retelling.

Families And Twins

What Runs in Families, and Why That Is Not Enough

A family study compares the rate of a condition among relatives of affected people. Against the rate in the general population. If the condition is genetically influenced, closer relatives should show higher rates. Such studies reliably find elevated rates, and that is genuine information. It establishes that the condition runs in families. And it tells a clinician that family history is worth asking about. But there is a confound it cannot remove. Families share more than genes. They share housing, income, neighborhood, diet, stress, parenting, language, religion. And the experience of living alongside someone who is unwell. Any one of those could produce familial clustering with no genetic contribution at all. A twin study compares two kinds of twin pair. Monozygotic twins develop from a single fertilized egg, sharing essentially all genetic material. Dizygotic twins develop from two separate eggs. They share on average about half of the genetic variation that varies between people. The comparison is elegant. Both kinds of pair are the same age. Born to the same parents, and usually raised in the same household. So what differs systematically is the proportion of genetic material shared. Concordance is the probability that, given one twin has a condition, the other does too. Higher concordance in monozygotic pairs is the standard evidence for genetic contribution. It rests on the equal-environments assumption, and that word assumption matters. Identical twins are often treated more alike, and share more friends and activities. Researchers have tested the assumption, and the results broadly support it without settling it.

Designs

Each Buys Information at the Price of a Limitation

An adoption study examines people raised apart from their biological relatives. Which separates the two things a family study confounds. If adopted children resemble their biological parents more than their adoptive ones. Genetic influence is indicated. If the reverse, environmental influence is. The design is powerful, and it carries its own limitations. Adoptive families are screened, and are therefore unrepresentative. Typically more stable and better resourced than average. That restricts the range of environments sampled. And it can make environmental influence look smaller than it is. Placement is also often not random, since agencies may match children to families. And the prenatal environment is shared with the biological mother, and is not genetic. The strongest available design combines the two. Monozygotic twins reared apart. Such pairs are rare, so those studies are small and carry the sampling limits. Worth remembering, since they are also the studies most often cited in popular accounts. Each buys a specific piece of information at the price of a specific limitation. No row establishes that a genetic variant causes a condition in a person. These designs answer questions about variation in populations.

Molecular

What a Polygenic Score Can and Cannot Do

The designs so far establish that genetic influence exists. Without identifying any of the genes involved. Molecular methods attempt the identification. A genome-wide association study compares the frequency of very many variants. Between people with a condition, and people without it, looking for differences. It requires very large samples, precisely because the effects are small. Its results are what established the polygenic picture. An associated variant is not necessarily a causal one. It may simply sit near a causal variant on the chromosome. A polygenic score sums the small effects of many variants into a single number. Indicating where a person sits on a distribution of estimated genetic risk. What such scores can currently do is predict, weakly, average differences between groups. What they cannot do is predict whether a particular person will develop a condition. The distributions overlap enormously. A person with a high score is more likely than not to remain well. And a person with a low score can become unwell. One further limit. Most scores were derived in populations of European ancestry. They predict considerably worse in others. That is a limitation of the samples, and not a finding about populations. Which is why consumer genetic testing reports about psychological traits require caution. They rest on exactly the weak individual prediction just described.

Defined Precisely

Heritability Is About Variation, Not About You

Now heritability, defined precisely, because the definition is doing all the work. Heritability is the proportion of the variation in a characteristic. Within a specific population, at a specific time. That is statistically attributable to genetic variation within that population. It is about variation. About a population. At a specific time. And it is statistical. Now note what it does not contain. It says nothing about any individual. It is not a proportion of a person's condition. And it is not a measure of how genetic something is in any everyday sense. Take an illustration. Suppose a condition has a heritability of zero point four zero. That means about forty percent of the differences between people in that population. Are statistically associated with genetic differences between them. It does not mean forty percent of any person's condition is genetic. That is not a coherent statement. And it does not mean forty percent of people with the condition inherited it. Heritability also changes with how much environments vary. Where everyone shares a similar environment, environmental variation is small. So a larger share of the remaining differences is attributed to genes, and heritability rises. Which means high heritability can indicate an equal environment. As readily as it indicates a strong genetic effect. And now the point that matters most, stated without hedging. Heritability within a population says nothing whatever about differences between populations. A characteristic can be highly heritable within each of two groups. While the average difference between the groups is entirely environmental. That is the specific error historically used to give cover to claims about group differences. And it is a matter of logic rather than of politics.

Corrected

Six Misreadings, and What to Say Instead

Six misreadings recur, so take them one at a time. This condition is forty percent genetic. Wrong, because heritability describes variation between people. Not a proportion of anything within a person. Her condition was forty percent caused by her genes. Wrong, because it applies a population statistic to an individual, where it has no meaning. High heritability means the environment does not matter. It confuses the source of variation with the possibility of change. A highly heritable characteristic can be substantially changed by an environmental intervention. Heritability is fixed for a condition. Wrong. It depends on how much environments vary in the population studied. It is heritable in both groups, so the difference between them is genetic. Wrong. Within-group heritability carries no information about between-group differences. And finally, high heritability means a strong genetic effect. Wrong. A uniform environment reduces environmental variation. Which inflates the proportion attributed to genes. Notice that four of those six share one root. Treating a statement about variation between people as a statement about something inside a person.

Two Relationships

Interaction, and Correlation - Persistently Confused

Two relationships between genes and environments recur, and they are constantly confused. Gene-environment interaction occurs when the effect of an environment depends on genotype. Or equivalently, when the effect of a genotype depends on the environment. Suppose two genotype groups show almost identical outcomes when adversity is low. While at high adversity, one group shows much worse outcomes than the other. Neither the genotype alone nor the adversity alone predicts the outcome. The combination does. That is what interaction means. Gene-environment correlation is a different thing entirely. It occurs when genotype influences which environments a person encounters. So the two are not independent to begin with. Three routes are conventionally distinguished. Passive. Parents provide both the genes and the environment. So a child inherits a predisposition and a household shaped by the parents who have it. Evocative. A person's characteristics draw particular responses from other people. An irritable child evokes different parenting than a placid one. Active. As people gain autonomy, they select environments matching their dispositions. An anxious adolescent may avoid the very situations that would reduce the anxiety. The distinction has a clean test. In interaction, the environment arrives independently of the genotype, and the two combine. In correlation, the genotype affects which environment arrives in the first place. And the practical consequence is large. Correlation means an apparently environmental finding may be partly genetic in origin.

Mechanism And Origin

Epigenetics, and Why Vulnerabilities Persist

Epigenetics is the study of changes in gene expression. Which genes are switched on or off, and how strongly. Changes that do not involve any alteration to the DNA sequence itself. The distinction is between the text, and which parts of it are being read. A way experience could produce lasting biological change. Stress, nutrition, and early care are associated with epigenetic differences. And those differences can persist. It is also among the most overclaimed topics in current popular science. Much of the strongest evidence comes from animal studies. Human studies typically measure marks in blood, while the tissue of interest is the brain. And claims about inheritance across human generations are contested. Now a different question. If these vulnerabilities reduce functioning, why do they persist? Trait-level advantage. The underlying traits may be useful at moderate levels. Vigilance, caution, and low mood after loss may each carry benefits mid-range. Mismatch. The environments in which human responses developed are not the ones we inhabit. On that account the response is not faulty. The environment changed faster. And normal defensive systems misfiring. A system costly to under-use and cheap to over-use will be tuned to fire often. Which predicts frequent false alarms as normal operation, rather than as failure. But state the testability problem as a real limitation, not a footnote. Evolutionary explanations are constructed after the fact, about events nobody observed. A flexible enough account can be produced for almost any characteristic. Including, if the finding had gone the other way, its opposite. An explanation that could accommodate any result forbids nothing.

A Composite Family

Three Generations, and Why the Pattern Settles Nothing

One composite family, constructed for instruction, describing no real family. Priya is twenty-six, and is assessed for persistent low mood that began at twenty-two. Her mother had a long period of similar difficulty in her thirties. Her maternal grandmother was described within the family as having had bad nerves. And rarely left the house in later life. Priya's older brother has had two episodes of low mood. She describes her childhood as marked by her mother's illness. Including periods when her mother could not get out of bed. And when Priya took on the care of her brother. Now, why does that pattern settle nothing? Three generations show the difficulty. That is what genetic transmission would produce. It is also exactly what environmental transmission would produce. The family shares genes. It also shares a household, an income, and a neighborhood. And critically, the experience of growing up alongside a parent who was unwell. Priya's childhood caregiving role is an environmental exposure. Transmitted by the condition itself, rather than by DNA. Gene-environment correlation is present too, in its passive form. Her mother provided both the genes and the environment shaped by having the condition. What evidence would separate them? An adoption design, or a twin design. None of these is available for this family. And none would answer the question the family is actually asking. Two points to reproduce. The family pattern is genuinely informative. It establishes familial transmission, and Priya's presentation should be reviewed. It simply cannot distinguish the route, and more family detail would not help. And the clinical implication does not depend on which route predominates. Which is a reason to describe the pattern as raised risk, rather than as an inheritance.

About Abnormal Psychology at UPI Study

Abnormal Psychology (Psychology 180) is a self-paced online course of 97 video lessons in 25 chapters, from the main theoretical perspectives and clinical assessment to anxiety, mood, psychotic and personality disorders, treatment and the law. It carries a NCCRS credit recommendation for 3 semester credits.

Every lesson transcript is free on pages like this one, and the videos are free with a free UPI Study account. Enrolling adds a practice quiz after each lesson, the graded work and final exam that earn the credit, lifetime access, and an official transcript on completion. UPI Study has helped over 130,000 students with college credits. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.

Enroll now → Course details Download the syllabus All Psychology and social science courses Every course with PRO →

Why take it here

Why students take Abnormal Psychology with UPI Study

Understand mental health disordersAbnormal Psychology covers how psychological disorders are defined, assessed, explained and treated. It is a core course for psychology majors and useful for nursing, social work, counseling and criminal justice.
Credit at a fraction of the priceThe same 3 credits can cost $750 or more at a US college. Here it is one payment, self-paced, with lifetime access.
Finish your degree soonerTransfer credits reduce the courses you still owe, so you can graduate earlier or lighten a heavy semester.
Strengthen an applicationA completed college-level course on an official transcript shows admissions and scholarship committees you can do the work.
Study on your own scheduleNo start dates, no deadlines. Watch the lessons, practice, and take the final when you are ready.
130,000+students helped with college creditsACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.

Where the credit goes

Universities that work with NCCRS

Abnormal Psychology carries a NCCRS credit recommendation. These universities work with NCCRS; some are UPI Study's signed partners. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.

Thomas Edison State UniversityThomas Edison State UniversitySigned partnerUniversity of the PeopleUniversity of the PeopleSigned partnerCharter Oak State CollegeCharter Oak State CollegeSigned partnerStrayer UniversityStrayer UniversitySigned partnerColumbia Southern UniversityColumbia Southern UniversitySigned partnerIndiana TechIndiana TechSigned partnerPenn StatePenn StateWorks with NCCRSSouthern New Hampshire UniversitySouthern New Hampshire UniversityWorks with NCCRSSUNYSUNYWorks with NCCRS +many more cooperating universitiesFind my college →

Find my college → Not listed? Ask Athena in the corner of this page, or book a 15-minute call.

FAQ

Questions students ask us

About How Genetics and Evolution Shape Psychological Vulnerability

What does heritability actually mean?
Heritability is the proportion of variation in a characteristic within a specific population, at a specific time, that is statistically attributable to genetic variation in that population. A heritability of 0.40 means about 40 percent of the differences between people in that group are associated with genetic differences. It does not mean 40 percent of any one person's condition is genetic, and it says nothing about differences between populations.
Is there a gene for depression or anxiety?
No, there is no single gene for depression, anxiety, or other common psychological conditions. These conditions are polygenic, meaning many genetic variants each contribute a very small effect, and no single common variant raises risk by a clinically meaningful amount. The same variants are also implicated across several conditions. Headlines about finding the gene for a condition usually describe one small-effect variant among many.
Why are twin studies used to study genetic influence on mental disorders?
Twin studies compare identical twins, who share essentially all their genetic material, with fraternal twins, who share on average about half of the genetic variation. Both types are the same age and usually raised together, so higher concordance in identical pairs suggests genetic influence. The design rests on the equal-environments assumption, which research broadly supports without fully settling, since identical twins are often treated more alike.
What is the difference between gene-environment interaction and gene-environment correlation?
Gene-environment interaction occurs when the effect of an environment depends on a person's genotype, so the combination predicts outcomes better than either alone. Gene-environment correlation occurs when genotype influences which environments a person encounters. Correlation can be passive, as when parents provide both genes and a home, evocative, when traits draw responses from others, or active, when people choose settings that match their dispositions.

About the Abnormal Psychology course

How many college credits is Abnormal Psychology?
Abnormal Psychology (Psychology 180) carries a NCCRS credit recommendation for 3 semester credits. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.
How much does Abnormal Psychology cost and what is included?
$250, paid once, with lifetime access: all 97 video lessons, a practice quiz after each, the graded work and final exam, and an official transcript when you finish. Students who want several courses choose UPI Study PRO instead.
Which universities accept UPI Study credits?
UPI Study has signed partner agreements with universities such as Thomas Edison State University, University of the People and Charter Oak State College, and works with universities such as Penn State, SNHU and SUNY. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities. Use "Find my college" on this page to check yours.
How long does Abnormal Psychology take?
It is self-paced with no deadlines. The 97 lessons total about 24 hours of video; most students add reading and practice time and finish in a few weeks to a few months, on their own schedule.
How are the exams done?
Quizzes and assignments are online in the course. The final exam is taken online with proctoring; a person on the UPI Study team checks assignments, usually within two days.
How do my credits reach my college?
When the course shows 100% complete, request the official transcript from your dashboard. It is sent to the college you name, and you can send it to more than one. Check your college first with "Find my college" on this page, or ask Athena.

Ready for the full course? Earn 3 credits in Abnormal Psychology.

97 video lessons, a quiz after each one, and the graded work that earns the credit. Recommended for credit by NCCRS.

Enroll in Abnormal Psychology →Download the syllabusEvery course with PRO →
Abnormal Psychology
3 credits · NCCRSEnroll →