See the U.S. workforce grow from 1970 to 2012 in this animated GIF infographic. (Miss a fact or figure? Don't worry, the image replays every 30 seconds.)

In 1970, there were about 140 million Americans eligible to work. That includes anyone over the age of 16 who is not in prison or in the active military -- what the Bureau of Labor Statistcs calls the civilian noninstitutional population.

Of those 140 million Americans, 78.5 million were employed full- or part-time. Another 4.4 million were unemployed and looking for work. Only those who have looked for a job in the past four weeks are officially defined as "unemployed."

The remaining 55 million fell into a category known as "not in the labor force." That group includes students, military, retirees, people caring for kids or elderly relatives and the unemployed who have fallen out of the workforce because they are no longer counted as such by the government or they aren't looking for a job.

A changing workforce
Fast forward to September 2012 and the U.S. employment picture looks very different. Simply put, there are a lot more people: more people working, and more people not working. About a 100 million more to be exact. (Spend 30 seconds with the animated GIF infographic above to see for yourself.)

The New York Times wrote about this shift in the employment-population ratio on Wednesday in its Economix Blog, attributing the change in part to the Baby Boomer generation.

Thanks in part to the baby boom, the employment-population ratio understates the amount that the economy has recovered.

Before the recession began, 63 of every 100 people age 16 and over were employed. The percentage plummeted to 58.4 by the fourth quarter of 2009 and hasn’t moved far from there since.

This is most apparent when you compare the workforce in September 2005 to September 2012.

The number of people employed is about the same: roughly 143 million. But the umemployment rate is 7.8 percent now compared to 5 percent in 2005. And there are 17 million additional Americans eligible to work. Some of them are considered "marginally attached" to the labor force, meaning they are available for work and want to work; they have looked for a job in the past year, but have not looked in the past four weeks. A further subset of the marginally attached are defined as "discouraged workers." These people say the reason they haven't looked for work in the past four weeks is that there are no jobs available for them.

So what implications does this changing employment picture have on the economy? And what happens to these people?

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Follow Matt Berger at @byMattBerger