- The Dutch: Back in the days there were no dykes, the Netherlands was mainly a big swamp. In these times it was difficult to get food and have agricultural activity. Due to the scarcity of food, people had to be greedy (or rather: efficient) to not to starve. The Golden age also made us known as greedy and sly people, I don't think that needs an explanaition.
- The Jews: In the medieval age, Jews were excluded from guilds, because of their religion. That lead to them choosing something else, such as trading and financial services, making them known with money.
- The Scots: Same story as the Dutch. The northern parts of the (nowadays called) UK were much harder to live in. Not to mention that the creation of Dagobert (Mc)Duck (Scrooge McDuck for the US) didn't make matters better.
Read that in Quest (Jan. 2014 edition).