Ask HN: How much would changes in tax policy affect entrepreneurship?

I don't have a particular horse in this race, but as an academic far removed from the world of business, let me ask the HN community something I've long been curious about.

How much would/do changes in tax policy affect entrepreneurship and willingness to take risks?

As an example, there was a recent controversy in San Francisco as to whether to offer Twitter an exemption to its payroll tax, to entice them to stay in the city.

More generally, we hear proposals to raise the top marginal income tax rates back to 39.6%, or higher, or to tax capital gains as regular income, or what have you. The most common argument advanced against this that I hear is that it would stifle business, entrepreneurship, and job growth.

Would it? If you started (or intend to start) a startup, would you still have done it if any windfall was subject to, say, 65% taxation?

Not trying to start an argument, I promise! Indeed, the perspectives and motivations of individual founders or potential founders are what they are, and as far as this debate goes, they seem to be the closest thing we have to hard facts. Hence my curiosity.