Researchers at Yale University did a study of prospective students applying to MD/PhD programs. It found (as if this is news) that these programs do not attract a wide diversity of students, especially from lower-income backgrounds.
Between 2014 and 2019, applicants from families with higher household incomes were accepted at increasingly higher rates, a trend not found among other income brackets.
Yale researchers lament the lack of diversity in MD PhD programs, which is decreasing slightly.