<p>It depends on the PhD you’re talking about.</p>
<p>In certain doctoral programs, almost everybody majored in something else in undergrad for the simple reason that certain disciplines are not universally taught at the undergrad level. Perhaps the best example is doctorate in business (management). The vast majority of business doctoral students did not major in business for undergrad simply because many undergrad schools do not even offer a business undergrad major, and of those that do, the vast majority of them are not particularly strong. There are only a few schools with highly respected undergrad business programs - Penn, MIT, Berkeley, Michigan, and a few more. To give you an example, to my knowledge, I don’t think there is a single business doctoral student at Stanford who actually majored in business for undergrad. At HBS, there is only a handful.</p>