"twin studies fail to separate the effects of genes and the prenatal environment. This failure casts doubt on claims of the relative effects of genes and environment on intelligence, psychiatric disorders, personality and other psychological variables, and other conditions."

https://www.psychologytoday.com/gb/blog/looking-in-the-cultu...

"Although many twin studies have been conducted (which is quite an understatement; there are almost 9,000 hits for “twin study” on PubMed!), there have long been critics who argue that they are scientifically worthless."

Smith, Jinkinson. (2020). The debate over twin studies: an overview. http://dx.doi.org/10.22541/au.159674847.78026661

"Because heritability is defined by both genetic and environmental influences, it is not a fixed characteristic of a disease or trait, but a population-specific estimate, analogous to, for example, the mean height, cholesterol level or life expectancy in a population. It also cannot be interpretated at the family or individual level."

Kaprio J. (2012). Twins and the mystery of missing heritability: the contribution of gene-environment interactions. Journal of internal medicine, 272(5), 440–448. https://doi.org/10.1111/j.1365-2796.2012.02587.x