Twin Studies in AP Psychology

Step 1: Line up identical twins against fraternal twins

The basic design starts with a comparison. Identical twins come from one fertilized egg that splits, so their genes match. Fraternal twins come from two separate eggs, so they're genetically about as alike as ordinary siblings.

Both kinds of twins are usually born at the same time and raised in the same house. That means their environments are roughly equal, and the main thing that differs between the two groups is how many genes they share. If identical twins match on a trait like anxiety far more often than fraternal twins, genes are the likely reason.

This is the reasoning behind Topic 1.1, Interaction of Heredity and Environment, in Unit 1: Biological Bases of Behavior. The course asks you to explain how nature (genetic or predisposed traits) and nurture (outside influences like family and education) work together. Twin studies are one of the three methods the course names for studying genetic influence, along with adoption studies and family studies.

Step 2: Separate the twins to test shared upbringing

The strongest version studies identical twins reared apart. Their genes are the same, but they grew up in different families. Any similarity that survives that separation is hard to explain with shared upbringing. The best-known project of this kind is Thomas Bouchard's Minnesota Study of Twins Reared Apart.

A multiple-choice question could present a three-way pattern and ask what it suggests:

• identical twins reared together are the most similar
• identical twins reared apart are a little less similar
• fraternal twins are the least similar

The gap between identical and fraternal twins points to genes. A difference between identical twins reared together and apart can suggest environmental influence, although their experiences may differ in several ways, such as when they were separated and what kinds of homes they were placed in. The right answer says both heredity and environment contribute. Picking "genes alone" or "environment alone" ignores half the data.

Step 3: Read the leftovers as nurture, and watch for interaction

Twin data can also show genes and environment depending on each other. Take musical skill. Identical twins may be more alike than fraternal twins, but in a culture with no musical training, even identical twins would show little skill. A genetic predisposition only shows up when the environment gives it a chance.

That is exactly what the course means by "interact." When a question describes a trait that is partly heritable but needs the right experiences to develop, the answer is the interaction of heredity and environment.

This step also connects to a warning in Topic 1.1. Some theorists have used claims about heredity to justify eugenics, the discriminatory idea of controlling who reproduces. Overstating twin findings as "it's all genetic" is the same kind of reasoning error on a smaller scale.

Step 4: Check what a twin design can and can't prove

Twin studies are correlational. Researchers can't assign people to be twins or randomly assign their genes, so they estimate genetic influence without proving cause. Their numbers describe variation across a group of twins. They don't tell you how much of one person's trait came from genes.

Several confounds can make genes look stronger than they are. Identical twins share a more similar prenatal environment and may be treated more alike by parents and peers than fraternal twins are. Twins "reared apart" are sometimes placed in similar homes, or were separated after spending time together.

These limits matter on the free-response section. In Question 1 (Article Analysis), a summarized twin study might ask you to identify the method, the variables, or whether the findings generalize, since twins are a specific population. In Question 2 (Evidence-Based), a twin study could be one of three sources on a trait's origins. A strong answer cites the specific comparison (identical vs. fraternal, together vs. apart) and explains what that comparison shows about heredity and environment.

Twin study or adoption study: sorting the scenario

Both methods try to separate genes from environment, so they get mixed up. A twin study changes the amount of shared genes by comparing identical and fraternal twins. An adoption study compares a child with two sets of parents: biological parents share the child's genes, and adoptive parents provide the environment.

A quick check helps here. If the stem compares how similar twins are, it's a twin study. If it asks whether a child resembles birth parents or adoptive parents more, it's an adoption study. Separated-twin studies use both ideas, but they're generally called twin studies because the comparison is between twins.