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The Twins Project

How Many Twins?

About 32 out of every 1000 people is a twin.  Identical twinning seems to be due just to chance.  Some people have suggested that having fraternal twins is hereditary:  If someone else in your family had fraternal twins, then you are more likely than other people to give birth to fraternal twins, too.  The percent of people who are twins differs by race and ethnicity.  In the U.S., twins are most common in African-Americans (36.8 per 1000) and least common in Hispanic/Latino Americans (21.8 per 1000).  Over 20% of births to moms over the age of 45 were twins, but only 2% of births to moms under age 24 were twins.  The states with the highest rates of twins are Connecticut, Massachusetts, and New Jersey, and the state with the lowest rate is New Mexico.

Identical or Fraternal?

All opposite-sex twin pairs (one boy and one girl) are fraternal twins.  Opposite-sex twin pairs make up about 1/3 of twin births.  The Bobbsey twins, who were characters in a famous series of children’s books, were opposite-sex fraternal twins.

When twins are the same sex (both boys or both girls), it can be harder to tell whether they are identical or fraternal.  In fact, up to 20% of twins think they are fraternal when they are actually identical!  Often people are confused because they have heard that if the twins are born with two placentas (the organ that supplies a fetus with oxygen and food), then the twins are fraternal.  In fact, almost 40% of identical twins are also born with two placentas.

The only sure way to tell whether twins are identical or fraternal is through DNA testing.  Twins’ blood or spit is used to get a sample of DNA, and then researchers test to see how similar twins are for a number of short bits of DNA that differ between people (called genetic markers). Usually, though, it is possible to tell whether twins are identical or fraternal based on how similar they are for their hair and eye color, their height and weight, and their facial features.  This works in about 95% of cases.  If distant relatives, new friends, and teachers can’t tell you apart, then you are probably identical twins!

Famous sets of twins include Ronde and Tiki Barber (both professional football players), Bob and Mike Bryan (professional tennis players and doubles champions), James and Oliver Phelps (actors who played the Weasley twins in the Harry Potter movies), Mary Kate and Ashley Olsen (actresses), and Barbara and Jenna Bush (daughters of former President of the United States, George W. Bush).

Links to Other Twin Projects

Danish Twin Registry

St Thomas UK Adult Twin Registry

The Centre for the Study of Multiple Birth

The Finnish Twin Cohort Study

The Twins Foundation

The Queensland Institute of Medical Research

Institute of Behavioral Genetics, University of Colorado

International Society of Twin Studies

Italian Twin Registry

Harvard Twins Study

Multiple Birth Foundation (UK)

Netherlands Twin Register (NTR)

Sri Lankan Twin Registry

Swedish Twin Registry

The Southern California Twin Project

University of Washington Twins Study – Chronic Fatigue Syndrome

Virginia Institute for Psychiatric and Behavioral Genetics

Northern California Twin Registry

The Massachusetts Twin Registry

Vietnam Era Twin Registry

Wisconsin Twin Research