Ask HN: Who says tax breaks create jobs?

While plausible, I would love to see some empirical evidence that tax breaks do, in fact, create jobs. Where is it? And where is the evidence against the Keynesian idea that the added personal income from a tax break will (basically) just be stuffed under the mattress in a bad economy (in bad investment climates, investors prefer to remain liquid).

My bias is toward Keynes, but I would like to actually have data, not just ideologically driven conjecture to base that on or to change it.