I think he's on to something. So I used his data source (FRED - Mark has the links) and made my own graphs. I like to put time series data on a log scale, to see if there is a constant growth rate. Here is a simple, clean graph, on a log scale, with a trend line (Mark used an exponential trend line on a linear scale.)
Not too surprisingly, this is pretty similar to GDP/Cap. Also, there might be a break point around 1980, as with GDP. Let's have a look.
The years up to '80 are in green, from '80 on in red. Now there are 3 trend lines. The green line is quite different from the other two. The post '80 line (red) has the lowest slope.
Lane Kenworthy, looking at income data, puts a break point at 1973. Let's give that a try. Up to '73 is in blue, post '73 in red, total data set trend line is in yellow.
Here, I think the 1973 breakpoint is even more convincing than in LK's graph.
Stagflationary Mark highlighted the last 10 years to emphasize the current stagnation. Let's do that, too.
Here we can see the gap of the current condition from the long ago trend line (blue.) Even worse, we're falling away from the total data set trend line (yellow.) Even still more worser, we're falling away from the post '73 trend line. Mark concludes:
Real disposable personal income growth is currently not back on track. I can pretty much assure you of that.
Like I said, I think he's on to something. And doesn't that golden age look golden!