A- wrote:
Are folks with Ph.D.s generally more intelligent than the overall population? Or, perhaps, they're just more driven/motivate to stick with the grad program to the end?
For some good data on this, scroll down to the third graph ("Project talent: general, math, verbal, and spatial aptitude) in this link:
http://qz.com/334926/your-college-major-is-a-pretty-good-indication-of-how-smart-you-are/There's lots of information there, but the key point is the difference between bachelors, masters, and PhD recipients in every specialty. At each step, the filter is a little narrower, so that average "intelligence" (or whatever you want to call these scores) is a little higher.
Does that mean EVERY PhD student in a given specialty is smarter than EVERY bachelors student? Of course not. There are some relatively thick PhDs (or, to be nicer about it, people whose persistence is their strongest point), and lots of very smart people who haven't pursued higher education. But if, like the OP, you're asking about general trends, then yes, PhDs are generally smarter than people without PhDs.
Note, for example, that the PhDs in social sciences and humanities actually score higher IN MATH APTITUDE than the bachelors recipients in math and physical sciences. That's a pretty solid indicator that a PhD is a general marker of intelligence. Similarly, check out the verbal scores of the math and physical sciences PhDs: way higher than even humanities masters degrees.