A limitation of some twin studies is that they sometimes lack controls and ignore disorder subtypes. Which option best reflects this limitation?

• A. They always have perfect controls
• B. They ignore genetic influence
• C. They sometimes lack controls and ignore disorder subtypes
• D. They prove genetics solely determine traits

Answer: (C)

Limitations of twin studies lie in design weaknesses that can bias conclusions about genetics. When studies don’t include solid control comparisons or fail to separate disorder subtypes, the results can blur what’s due to genes versus the environment and can miss how different subtypes may be influenced by different genetic factors. If controls are weak, shared environments or other confounds can be mistaken for genetic effects. If disorder subtypes are ignored, you treat diverse conditions as one, obscuring varying heritability across subtypes. That’s why the option describing both lacking controls and ignoring subtypes best captures the limitation.