I'm not sure about the hereditary bit (whether culture or risk aversion can b passed through genetics), but the culture bit has a lot of basis. Just look the the whole 'American Dream'. The US has been sold as a place where anyone can make it with some smarts, work, and perseverance. That attitude was ingrained in the country through its formation and then its expansion. Look at the silver rush in CO, gold rush in CA, oil boom in Texas, financial boom in NYC, and tech boom in SV. People have been coming to the US from all over the world to get rich since the country was formed.

One thing to remember is that the US is a relatively young country. It may be that all young countries have a disproportionately large number of risk takers and that over time that portion shrinks.