What does 60% “heritability” mean?

Technically speaking, heritability estimates the amount of phenotypic variation (i.e., the variation in a certain trait, such as having RA) that is caused by variation in genes in a given population. An easier way to understand the concept is to consider the extremes. A heritability of 100% would indicate that RA (or the trait in question) could be 100% attributed to genetics and 0% to environmental factors. In contrast, a heritability of 0% would indicate that genetics were not involved whatsoever in developing the disease. It is important to note that “60% heritability” does not mean that, in any individual with RA, 60% of the cause can be chalked up to genetics. (Heritability is defined only at the population level, not at the individual level.) It also does not mean that 60% of individuals with RA have RA because of their genetic makeup.