In the US, statute requires that the inventor(s) be identified on each patent application. Improper inventorship, especially when done with deceptive intention, can invalidate a patent.
In the Big Companies, the definition of inventor is a very important issue to the inventors themselves. Patents are fodder for resumes as well as a bonus program, sometimes putting thousands of dollars in an inventor’s pocket. It is hard to underestimate the emotional turmoil that can be stirred by adding or omitting names to a patent application.
An inventor conceives of an idea and develops it to the point that someone can practice the invention. Sometimes this process occurs instantly, when the inventor sketches the widget for the machinist to make, and sometimes it occurs over a great deal of time, as failure after failure happens before the breakthrough.
In my experience as an inventor in Corporate America, each person has their own idea about who is an inventor. The typical scenario happens like this: a big meeting is called to solve a particular problem. People start suggesting different ideas and finally someone describes an elegant solution, to which everyone agrees. The inventor described the solution to everyone present in enough detail that they could all go implement the solution.
However, there are always several people at the meeting who see the light right after the inventor described it. Many times, these people think they invented it, too. Sometimes, these other people contribute one very minor suggestion, but still think they are inventors.
Since there is money and prestige involved in the Corporate World, getting named on an invention is a very emotional issue for many people.
The bottom line: the inventorship is determined by the claims of the invention. If an inventor contributed one limitation to one claim by conceiving and expressing that limitation, he or she should be named.
The key to invention is the conception of the idea. There is an incredible feeling inside when you conceive of a solution that no one had ever seen. Often, a similar, yet different feeling happens in someone when they are told of this inventive solution. Sometimes that person gets mixed up and thinks that they invented the solution. (Somehow, these people tend to be in management, but I digress )
Usually, there is one person who truly conceives and invents an idea. This person is usually the goose that is laying the golden eggs. Adding countless other people as co-inventors to the true inventor’s patents is minimizing the inventor’s contribution and threatening to kill the goose.