Something happened in the mid-70's
which caused inequality to increase steadily thereafter. There's a theory (not mine, but I think there's something to it) that the wealthy basically control how much inequality there will be, through their control of finance, manufacturing, government and government regulation. The only periods when the wealthy will allow an equitable distribution of wealth is when they feel that their own control is at risk, and that this only occurs during wartime when there is a draft and Americans are being sent overseas to die. The theory holds that when this happens, the wealthy fear that the American public will unify and threaten their control and even their existing wealth. Therefore, during these periods only, they consent to higher taxes on themselves and greater wealth distributions to the non-wealthy. But as soon as the crisis ends, so does their willingness to share.
So what happened in the mid-70s? The Vietnam War ended in April 1975. The last person was drafted in 1972, but it makes sense that it took a few years for inequality to show a definite upward trend.
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