Just think of national optimism, Hitler did. He motivated an entire nation to do some (initially) incredible things. (Later he ruined his good work in my opinion)
On a similar note see how media can affect the entire mood of a nation negatively or positively. Optimism and depressions are both fed by minds focussing on certain things. The results are truely tangible though.
Think of the ideas such as 'democracy'. It was spread by people being aware of it and them wanting it. Its still spreading now yet it is just an idea.
Same with fundamentalist Islam, its growing because people are feeding it by thinking of it.
These are group effects but they also apply to us at an individual level.
We tend to find what we are looking for, we find what what we hold in our minds. Thats perhaps the simplest way of putting it.
