Why do two children from the same family turn out very differently? If two people have the same biological parents, grow up in the same home, live in the same town, go to the same schools, and have similar experiences, what explains the differences in personality?
Two Very Different Brothers
When investigating sibling genetics, we’re all familiar with cases in which two people from the same family are different. One daughter teaches in a prestigious university, wins awards for her research, does community volunteer work with disadvantaged children, is a talented artist, and is a good wife and mother—while her sister, just a couple of years different in age, didn’t finish high school, can’t hold a job, has had a series of failed marriages, and has been arrested numerous times for shoplifting. What’s going on here?
One real-life example is former President Jimmy Carter and his brother, Billy. Jimmy Carter graduated from the U. S. Naval Academy in the top 10 percent of his class, he was a successful businessman, served in the Georgia State Senate, was elected Governor of Georgia, and was elected 39th president of the United States in 1976. After he was defeated by Ronald Reagan in his bid for re-election, President Carter devoted his time to causes related to human rights, receiving the Nobel Prize in 2002.
This is a transcript from the video series Understanding the Mysteries of Human Behavior. Watch it now, on The Great Courses Plus.
Comparatively, Jimmy had a brother, Billy, who was quite different from the ex-president. Billy didn’t finish college. For a while, he worked on the family peanut farm before owning a gas station. He ran for mayor of his hometown, but he wasn’t elected. He did, however, have a little success as the spokesperson for “Billy Beer.” He didn’t manufacture the beer, but he was a spokesperson for the company, capitalizing on his image as a Southern, beer-drinking, good old boy—but even that success came about because he was the president’s brother. Billy sometimes seemed to be an embarrassment to the family, like the time he urinated in public on an airport runway.
There was nothing wrong with the man’s life, but Jimmy and Billy were so different that it’s hard to understand how they could be brothers from the same family, who grew up in the same town in similar circumstances.
A Genetic Explanation?
To unravel the mystery of how this can happen, let’s start with the genetics. At the time of conception, the 23 chromosomes in the mother’s egg and the 23 chromosomes in the father’s sperm combine, resulting in a baby with 46 chromosomes. Half our genes are from our mother and half of them are from our father. If there are any siblings, your brothers and sisters also inherited 23 chromosomes from each parent. On average, children with the same biological parents share about 50 percent of their genes with each of their brothers and sisters.
But, unless you have an identical twin, your brothers’ and sisters’ chromosomes have different combinations of genes than yours do. The fact that they share half of your genes might suggest that two children with the same parents ought to be about halfway alike—not the same, but not completely different either—but that’s not true.
Personality characteristics are rarely due to a single gene. There’s not a gene for conscientiousness, leadership ability, criminality, or any other psychological characteristic. These complex patterns of behavior are affected by particular combinations of genes acting on the development of the brain and other parts of the nervous system. One sibling could have several genes that, together, predispose him or her to have a certain characteristic, but a sibling who shared half of those genes wouldn’t show any of that same characteristic at all. Having half the genes for that trait might not result in that trait.
Learn more about the relationship between genes and the environment
Unlocking the Genetic Code
Biologists often compare genes to a “code” that specifies how certain parts of the body are to be constructed. Using the metaphor of a phone number, if we’re calling a long-distance phone number, we need 10 digits—the area code and number—to reach the person we want to call. We need all 10 of those digits to get the person that we want. If even one of those 10 digits is changed, it changes everything. We get an entirely different person on the other end of the call.
In the same way with genes, changing just one gene in a sequence of genes can change the outcome entirely. Although two brothers may share roughly half of their genes, they don’t share some of the sequences of genes that are responsible for complex aspects of their personalities. As a result, they don’t appear to be as similar as people might expect based on the fact that we have half of our genes in common.
What this also means is that identical twins are often much more than twice as similar as ordinary siblings or as dizygotic, or fraternal twins, are. It’s easy to fall into the trap of thinking that, if identical twins share 100 percent of their genes and ordinary siblings and fraternal twins share 50 percent of their genes, then identical twins should be twice as similar as normal siblings. This fact isn’t true.
Imagine that we had four children in a family, two of which were identical twins. The twins may be very similar psychologically, but not resemble the other two siblings much at all, and the other siblings may also be quite different from each other.
In sharing 100 percent of their genes, the twins also share all of those unique combinations of genes that normal siblings don’t. To return to the phone number metaphor, the identical twins have the same 10 digits. The other siblings share five digits, but that may mean that they have completely different numbers and different sequences of genes that result in quite different personalities.
Learn more about the brief, fascinating history of genetics
One example of this phenomenon is known as emergenesis. Emergenesis occurs when a trait is determined by a particular configuration of many genes. That specific combination of genes then leads a person to display a particular characteristic that isn’t seen in the rest of the person’s family. The trait’s inherited—it’s influenced by genes—but there isn’t any hint of it in other family members.
If that person had an identical twin, the twin might be very similar, indicating that something was being inherited after all. It’s just that no other family member has the entire combination of genes—no other person has the same phone number—but the twin did.
One way that researchers can spot emergenic characteristics is when data show that identical twins are very similar, but fraternal twins are not. Imagine there are 100 identical twin pairs and 100 fraternal twin pairs, and we measure how extraverted each person is. Then the extraversion scores are correlated across the pairs of twins—separately for the identical twins and the fraternal twins—to see how similar the twins are to each other.
Learn more about why siblings can be so different
What we find is that the correlations are very high for identical twins but low for fraternal twins. This pattern suggests that extraversion is emergenic—it’s partly genetically determined but it doesn’t run strongly in families. One fraternal twin may be quite outgoing, and her twin brother or sister might be quite introverted—even though they share 50 percent of their genes.
A person’s emotional tone is also somewhat emergenic. Having an optimistic outlook, having control over one’s emotions, staying cool under pressure, and having a high capacity for happiness seems to require a particular combination of genes. Siblings can be quite different in their emotional tone. We all know families in which one person is happy and warm, while a brother or sister is unhappy and hostile. Identical twins tend to be much more similar in their emotionality.
Common Questions About Sibling Genetics
On average, the amount of DNA you share with a sibling is approximately 50 percent. Half-siblings share about 25 percent.
The reason that siblings only share 50 percent of their DNA, on average, is due to DNA swapping, which results in different gene combinations in the 23 chromosomes passed down from each parent.
First of all, genetics can account for sibling differences. Siblings usually only share 50 percent of the DNA passed down from their parents. Second of all, even if siblings attend the same school, they may hang out in different crowds, which in turn influences their personality. Finally, a child might purposefully act differently from his/her sibling in an attempt to form a unique identity.
Studies examining similarity in intelligence among siblings found that siblings closer in age tended to have more similar IQs than siblings that are far apart in age. This is probably because siblings closer in age are more likely to have similar environmental influences.