In twin studies, concordance rates are interpreted by comparing rates between identical and fraternal (non-identical) twins. If there is a significantly higher concordance rate among identical twins, the attribute of interest is the result of greater genetic similarity. If the concordance rates between identical and fraternal twins are more similar, the attribute of interest is the result of greater environmental factors. Unless there is a 100% concordance rate between identical twins, traits and disorders are understood to be at least somewhat influenced by both genetic and environmental factors.
Concordance rate is a statistical measure that describes the proportion of pairs of individuals that share an attribute, given that one already possesses this trait. A pair is considered concordant if they both possess an attribute of interest, and discordant if they differ. In psychology and other health-related sciences, the concordance rate is used to estimate the influence of nature (genetics) and nurture (environment) on a particular trait, disorder, or disease.
In health sciences, concordance typically means both members in a set of twins have the presence of a particular trait or disorder, such as ADHD. If only one individual in the pair has this diagnosis, the set is considered discordant.
When there is a high rate of agreement, or concordance, between sets of twins in a group, it is interpreted as evidence of a causal connection. For example, in a study, 99 out of 100 sets of identical twins are determined concordant for eye color. The concordance rate is 99%. From this, it can be estimated that eye color is almost entirely influenced by genetics rather than environmental factors.
In health-related sciences, concordance rate (also, concordance ratio) is commonly used to estimate the influence of nature and nurture on the development of a particular attribute or disease in an individual. This helps scientists, researchers, and clinicians untangle the influences of genes and the environment on specific traits or illnesses, such as major depression, height, or obesity in a population more generally.
The meaning of concordance rate is defined as "the proportion of pairs of individuals that share a particular attribute, given that one of the individuals has that characteristic." When looking for the concordance rate of a mental illness in twins, a pair is considered concordant, to agree, when both have this diagnosis. However, they are considered discordant, to disagree, when one does, and the other does not. In other words, the concordance rate is the proportion of pairs that agree in a study.
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There are two common types of twins, identical and fraternal. Identical twins occur when a single egg is fertilized by a single sperm (monozygotic) but splits into two after fertilization, resulting in two genetically identical embryos. Because the two resulting children have 100% of the same DNA, these twins only come in sets of males or females.
Non-identical, or fraternal twins, is defined as two different eggs that are fertilized by two different sperm (dizygotic). Fraternal twins are as genetically similar as ordinary siblings, sharing, on average, 50% of their DNA. However, twins that are raised together, unlike ordinary siblings, share the same gestational and home environment. This may sound counterintuitive but ordinary siblings, despite genetic similarities, are born to parents of different ages, may live in different housing types, experience different financial situations, and even be raised in completely different locations, or countries. Fraternal twins can come in sets of males, females, or one of each.
Twin studies offer a unique way to measure the relative contributions of genetics and the environment by comparing concordance rates across sets of fraternal and identical twins. Unlike ordinary siblings, when raised together, fraternal and identical twins share the same environment from conception through upbringing. If this is the case, it is assumed that sets of fraternal twins have about 50% shared genetic and 100% shared environmental components, while identical twins have 100% shared genetic and 100% shared environmental components.
Take a study looking at the incidence of schizophrenia in twins. If the illness is entirely caused by genetics, one would expect to find 100% concordance among identical twins, and somewhere closer to 50% among fraternal twins. More likely the case, if the illness is primarily (but not entirely) caused by genes, one would expect to find a higher rate of concordance, or agreement, between identical twins than fraternal. On the other hand, if schizophrenia is predominantly caused by the environment, one would expect to see similar concordance rates between the two groups because sets in both groups shared nearly identical environmental exposures and influences.
Before calculating the concordance rate, it is important to note that twin studies, like any methodology, have their limitations and critiques. Twin studies rely on several assumptions that must be accepted by researchers as true but have been challenged over the decades. These include: 1) there are only two types of twins, 2) the risk of receiving a diagnosis is the same for twins and non-twins, 3) twins grow up experiencing the same environment as one another, and 4) identical and fraternal sets of twins grow up in equal or similar behavior-shaping environments. For instance, it is thought that identical twins may be treated differently than fraternal twins. This could mean that identical twins have an even more similar environmental upbringing than fraternal twins, making a direct comparison more difficult.
Despite limitations or shortcomings, twin studies have helped shed light on the intertwined influences of nature and nurture. These studies do not identify the environmental or genetic causes of a disease per se, but they do point researchers in which direction to look.
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