I find this study on young amazon parrots very interesting. You must be a member to read the full text, but the abstract is more than enough to give any parrot (or other bird) keeper plenty of food for thought. Whether parrots should be kept in pairs for their own happiness, whether parrots kept in pairs have fewer behavior problems, whether paired parrots are better/worse companions, etc., are all issues that anyone keeping parrots for very long at all has come across, very likely in the form of a heated argument or at the very least a strongly voiced opinion, one that often verges on or fully embraces a warpath. All that debate comes with good reason, as there are legitimate points on both sides. However, I am not going to try to hide the fact that I think parrots are happier and better off if they can be kept with a compatible friend or mate!
Please, though, if you are thinking about going out and getting a friend for any of your parrots, from a budgie to a macaw, remember that like people, birds will not get along with every bird. They pick their friends as well as their mates. Also, if you parrot has been kept without any other bird for a long time, or was raised away from other birds, they may not recognize other parrots of even the same species as potential friends at all. The Amazon parrots in this study were parent raised (and note that they were being handled regularly by humans, and seeming to enjoy doing such), which makes a huge difference as they not only know they are parrots and know how to interact with other parrots. This is not to say, though, that it is impossible to get a friend for a single parrot! Even if they are not allowed to play together, just having another parrot in the room can be very rewarding for parrots. Parrots in the wild will not eat alone, sleep alone, or do anything else alone! With another parrot (I admit I think same species or compatible species from the same geographic area and/or similar size is best) they will have someone to be with all the time.
Studies like this are important because they show what could be possible if you applied knowledge to raise better parrots from the beginning, in this case keeping them in pairs from the start, instead of trying to fix problems later after they occur.
If a person has an untamed pair of parrots, either a small or large species, and much prefers tame cuddly parrots, they will be fully convinced the only good companion parrots are kept singly. On the other hand, many people have mated pairs of parrots, again from the very small to the very large, that are still wonderful and tame and very interested in people (and sometimes still enjoying scritches as well), so naturally they will have a very different view of things. My point? I try very hard not to put out statements based solely on my own experience. Most people only have experience in their own home with one parrot of a species, possibly two or three of that species. That really is very little to go on, and while individual experiences taken along with individual circumstances can be, collectively, very important and extremely useful, each piece on its own is not nearly so valuable, and can be very misleading. That makes studies like this, as rare as they are, even more important as a tool to cut through that cloud. The parrots are raised and kept under the same conditions, making comparisons possible that give the rest of us something to go on, and apply in our own homes.
And to reiterate, please go read this article! It is not very long, so really, please read it.