<p>Some departments, particularly at top places, essentially don’t allow it. For example, my son graduated Johns Hopkins with a BA in international studies, which is a second major administered by the Political Science Department. When he was applying for Ph.D programs (four years after graduating), the Department told him that they have an informal policy of never admitting Hopkins BA’s to the Ph.D program–not because they don’t think they are qualified (to the contrary) but because the department is relatively small and they don’t think it is good pedacogically to study with the same faculty for 6-8 years after 4 years of undergraduate work. Instead, they made calls and helped him gain admission to a fine program elsewhere. My son is now 4 years into his program and he now sees the wisdom of that policy.</p>
<p>FWIW, I’ve observed the above policy more in the humanities and social sciences. In the natural sciences, it seems to be more common to stay in one place through the Ph.D.</p>