Traditional methods can identify a specific genetic cause for about 18% of cases of "intellectual disability" (basically, mental retardation). State of the art genomics technologies have pushed that percentage to about 54% of case.
Some cases of intellectual disability of well known environmental causes (e.g. lack of folic acid and lead exposure), and there is some missing hereditary component of these conditions that do not have identified specific genetic causes. But, the proportion of the hereditary component that is "missing" because it lacks an identified specific genetic component is much smaller than for many other phenotypes known to have a hereditary source based on twin studies and similar considerations.