Spurring growth through Keynesian Economics?

From Robert J Barro (WSJ) - "Food stamps and other transfers aren't necessarily bad ideas, but there's no evidence they spur growth.":

Theorizing aside, Keynesian policy conclusions, such as the wisdom of additional stimulus geared to money transfers, should come down to empirical evidence. And there is zero evidence that deficit-financed transfers raise GDP and employment—not to mention evidence for a multiplier of two.

Gathering evidence is challenging. In the data, transfers are higher than normal during recessions but mainly because of the automatic increases in welfare programs, such as food stamps and unemployment benefits. To figure out the economic effects of transfers one needs "experiments" in which the government changes transfers in an unusual way—while other factors stay the same—but these events are rare.

Ironically, the administration created one informative data point by dramatically raising unemployment insurance eligibility to 99 weeks in 2009—a much bigger expansion than in previous recessions. Interestingly, the fraction of the unemployed who are long term (more than 26 weeks) has jumped since 2009—to over 44% today, whereas the previous peak had been only 26% during the 1982-83 recession. This pattern suggests that the dramatically longer unemployment-insurance eligibility period adversely affected the labor market. All we need now to get reliable estimates are a hundred more of these experiments.