rial). Because twin pairs tend to be raised in similar environments regardless 
of whether they are identical twins or fraternal twins, a higher social anxiety 
concordance rate in identical twins than in fraternal twins is thought to be 
evidence that genetics may have played a larger role in the development of 
the social anxiety (the term concordance rate  refers to the probability of one 
person having a particular problem if his or her twin also has the problem).
2. Adoption studies . In adoption studies, scientists interview both the bio -
logical parents and the adoptive parents of people who were adopted as 
children and who also have the particular problem being studied. If the 
researcher ﬁnds that social anxiety disorder occurs much more frequently in 
the biological parents of the socially anxious adoptees than in the adoptive 
parents, it suggests that genetics may be more important than environment. 
Although adoption studies have been used to study the role of genetics 
in various disorders and illnesses, this approach has not yet been used in 
research on social anxiety disorder.
3. Molecular genetics studies . In 2003, scientists completed the Human 
Genome Project, which involved mapping all of the genes in human DNA 
and determining the sequences of the 3 billion chemical base pairs that