How did we get 60/40 as the benchmark?

Spending time on asset allocation decisions, it seems that the standard for comparison is a portfolio of 60% equities and 40% bonds. Can you beat this benchmark? Is risk parity better than the 60/40 mix? Is the endowment model better than 60/40? How do you take tactical allocations tilts around the 60/40 benchmark? Everything seems to be compared to the gold standard, 60/40.

I wanted to ask a simpler questions. When did 60/40 take over the investment world? I have started a search for the answer and have not come up with any date for when this became the gold standard benchmark. It seems to have seeped into the investment world as a rule of thumb that is now  the best heuristic for what to do when you are not sure of any alternatives. From a paper by Russell Investments on asset allocation for non-profits as fiduciaries, I found:

And it gradually emerged that the average allocation across funds stabilized at roughly 60% in equities and 40% in bonds and cash. The expression 60/40 itself became a code-word for pension fund allocations. I remember one wit telling me, “It doesn’t matter what the pension question is, the answer is always 60/40!”

So why is this standard so good? It has done very well in the long-run because of the natural hedge between the two because of the negative correlation over long periods. There are periods when on an absolute basis it will not look very good, but the controlled volatility has made it a winner. 

We cling to this standard for some reason, yet on a conditional basis, it may not make sense. In a low rate environment, bonds will be a performance drag. when inflation is rising, bonds will also be a drag even if it provides diversification. Other benchmarks also have problems, yet there needs to be some well-defined goalposts for asset allocation. We seem to have a desire to change the goalposts from 60/40, yet we always return to this standard because there has been little agreement on a new standard. 

There needs to be a framework for determining the new standard and the idea of more asset classes as better may not be the right solution.