I'm still at a loss to explain Pennsylvania's decline at least relative to Ohio and Michigan. I think that @betarhoalphadelta 's theory that major Cities like NYC and Chicago draw people from the entire region not just the state explains PA's decline relative to NY and IL but, as I see it, Philly and Pittsburg aren't substantially less of a draw than Detroit, Cleveland, Columbus, or Cincinnati.
Here are some numbers to illustrate the change: In the late-1800s and early 1900s Pennsylvania's population was around half-again Ohio's and around two-and-a-half times Michigan's. The peak for PA relative to both OH and MI was in 1910 when PA's population was 161% of Ohio's and 273% of Michigan's. Ninety years later in 2000 Pennsylvania's population was only about 8% more than OH's and 24% more than MI's. It has recovered a little in the last two censuses but it is still only about 10% over Ohio's and 29% over Michigan's.
Ok, I think I have at least a plausible explanation so I want to bounce it off the group and see what you think:
The key is to look at Michigan. If you look at the chart back on the first page Michigan's population expanded pretty significantly relative to most of the others from about 1910-1980. For much of that time, Ohio grew right along with them. If you look particularly at the period from 1940-1980 Ohio's population went from about 51% of NY's to about 62% of NY's. Michigan's population went from 39% of NY's to 53% of NY's. If you just kinda eyeball the trend-lines for MI and OH from 1940-1980, had that continued they would both be more populous than IL and PA today.
The relative growth of MI and OH gets lost on the chart because their growth spurt is dwarfed by the massive population growth at the same time and a little later for CA, TX, and FL but if you ignored those three the biggest thing of note would be the relative growth of MI and OH.
I think it is the Auto-boom. Michigan obviously had an enormous growth in employment in the auto-industry in that era and Ohio has a LOT of auto manufacturing as well.
So NY and IL had major cities (NYC and Chicago) drawing in people from the entire region while MI and OH had massive growth in auto-industry jobs and PA had neither. They were the #1 Oil producing state until Texas passed them in the 1920s and in the ~100 years since their population has dropped from being a fairly close #2 behind only NY to being about equal with IL and only slightly ahead of OH.
Post-1980 the populations of OH and MI have drifted downward relative to NY right along with PA. That makes sense, foreign cars got big due to the oil crisis in the 1970s so ever since the auto industry hasn't contributed to big growth for MI and OH.
