Editing Twin study (section)

=== Extended twin designs and more complex genetic models ===
The basic or classical twin-design contains only identical and fraternal twins raised in their biological family. This represents only a sub-set of the possible genetic and environmental relationships. It is fair to say, therefore, that the heritability estimates from twin designs represent a first step in understanding the genetics of behavior.

The variance partitioning of the twin study into additive genetic, shared, and unshared environment is a first approximation to a complete analysis taking into account [[gene–environment correlation|gene–environment covariance]] and [[gene–environment interaction|interaction]], as well as other non-additive effects on behavior. The revolution in [[molecular genetics]] has provided more effective tools for describing the genome, and many researchers are pursuing molecular genetics in order to directly assess the influence of [[alleles]] and environments on traits.

An initial limitation of the twin design is that it does not afford an opportunity to consider both Shared Environment and Non-additive genetic effects simultaneously. This limit can be addressed by including additional siblings to the design.

A second limitation is that gene–environment correlation is not detectable as a distinct effect unless it is added to the model. Addressing this limit requires incorporating adoption models, or children-of-twins designs, to assess family influences uncorrelated with shared genetic effects.

==== Continuous variables and ordinal variables ====

While concordance studies compare traits either present or absent in each twin, [[correlation]]al studies compare the agreement in continuously varying traits across twins.