There's a whole bunch of factors that affect it. It's actually Eastern cultures that tend towards external factors (relative to us North Americans). But, in general, everyone focuses more on the individual factors than they should: it's called the Fundamental Attribution Error.
I've learned a bit about this recently in a Coursera class on social psychology. I tried to attach the lecture video but you need to be in the class to watch it. I recommend joining though - it's fascinating (and free).