Why are co-authors so important for the way I work? One reason is found in Adam Smith's famous story of the pin factory. Smith observed that the pin factory was so productive because it allowed workers to specialize. Research is no different--it is just another form of production. Doing research takes various skills: identifying questions, developing models, providing theorems, finding data, expositing results. Because few economists excel at all these tasks, collaborating authors can together do things that each author could not do as easily on his own. In manufacturing knowledge, as in manufacturing pins, specialization raises productivity. (The puzzle is why Adam Smith chose to ignore his own analysis and write The Wealth of Nations without the benefit of a co-author.)
The second reason I work with co-authors is that it makes my job less solitary. Research and writing can be a lonely activity. It is easy to spend endless hours with a pad and pencil or in front of a computer without human contact. Some people may like that kind of work, but not me. Arguing with my co-authors makes my day more fun.
The third reason I work with co-authors is the most important: a good co-author improves you forever. In the most successful collaborations, both co-authors learn from the experience. A co-author can help you expand your knowledge, improve your skills, and expose your biases. Even after the collaboration is over, you take these benefits with you to future projects. To a large extent, as I have grown older, my co-authors have become my mentors.
What do other academics think? Comments welcome.