Union membership and income distribution

In Who Killed American Unions, on the Atlantic's website, Derek Thompson speculates about a connection between technological change and the rise and fall of union membership, which has shrunk to just 12 percent of the US workforce. I was struck by this graph, comparing the rate of union membership with the middle class share of aggregare income: Writes Thompson: The apogee of the unions was also the apogee of the middle class, when it commanded more than half of total income. As the union membership rate dropped, middle class share of income fell, too....