The ecological fallacy is pretty easy to summarize: it is when you inappropriately assign a characteristic of a group to each individual member of that group.
This definition lets us know, there are valid applications of group definitions to members. For example, I can say that all bird lovers love birds. That is not a controversial statement.
But there are controversial examples, and they are quite insidious socially, I would argue.
For example, consider the rather anodyne observation that taken as a group, male humans are physically stronger than females. This is not a controversial statement.
But what about the statement that women cannot be Navy SEALs, since women are not as strong as men?
See the fallacy? Some logical moves from group to individual are valid, others are not. In this case, the identity "woman" tells us very little about the individual, in fact. And it is invalid to make conclusions that are not warranted from group descriptions to an individual estimation. That is the essence of the ecological fallacy.
Our minds would be much freer if we became more wary of the ecological fallacy. In some cases we are justified in making judgments about individuals based on their membership in a group (or set). In other cases, the exact opposite is the case. We are thinking fallaciously, and should not do it.
Now proceed to arguing, most men play melee classes, therefore if male, then bellicose. Etc. And so forth. have fun.
edit: that stupid Google dude's memo made me want to post this. It is textbook.
