Why do people behave differently when their identity is hidden?
The relative anonymity people enjoy (or suffer) on the Internet changes their thought processes. Other than the obvious abuses people can do when they believe that their deeds cannot be traced back to them, there are other, not-so-obvious subtle differences; even the individual himself may not be aware of it.
Take, for example, one of the more contrived examples, a DOTA game. For those not in the know, DOTA (otherwise known as Defence Of The Ancients) is a very popular mod (modification) based on Blizzard's Warcraft III: The Frozen Throne. It's a team game in which players choose heroes and try to destroy the enemy team's throne.
I will try to elucidate the following discussion in as neutral terms as possible, although there may be a few places in which I will use terms more familiar to those who already play the game. In this case, I apologize beforehand for the use of such jargon; although if you don't play the game already, I recommend you pick it up - it's fun! [ Those who already play, motivate those who don't play the game to play! ]
There exists within people a different mindset when playing an inhouse game, where everyone, or nearly everyone, knows each other, and when playing a public game (pubbie), which is where most of the players are strangers with each other.
In an inhouse, two of the more pertinent factors affecting a player's play are: Firstly, each player has a reputation, and secondly, most of the players are friends with each other.
Let's address the first issue. Players who have played with each other many times will be more or less aware of the relative strength and weaknesses of each of their allies and opponents. They will be more confident, and play far more aggressively, when facing a player perceived to be weaker, and will play more cautiously when facing one considered stronger. Furthermore, those considered strong will want to preserve their "reputation", and will want to do well. Thus, they will be far more critical on themselves as well as their teammates, since when their team mates do well, they do well (as is usually the case in most team games).
Now, I shall address the second issue. When players play often with each other, they will trust each other more, and they also will help each other more. This leads to a helpful "I know you've got my back" mentality. In a dangerous situation that is balanced between both sides, teams that have more trust in each other are more likely to take more risks (that may pay off) as they know that if they throw themselves into danger (a calculated gambit) in order to kill one or more opponents, they willl have allies to back them up. It is also more likely that in an inhouse game, you would track back in order to help your allies defend a "push" from your opponents, as you will be willing to help your friends in such potentially dangerous situations.
How about a pubbie then?
In a pubbie, there is unfamiliarity with allies/opponents. Furthermore, there is the factor of "you don't know me, I don't know you, after this game we won't play again" kind of mentality.
In a public game, at least for people like myself who consider ourselves more experienced, we consider the enemy to be inferior. This affects our play style greatly, in the sense that we may play far more aggressively or we will attempt to draw enemies into traps (for DOTA players: the most common trap early game in pubbies is to chase an enemy to his tower) far more. You may not be aware of this, but for most people, this is the subconscious instinct that they have in public games.
We also consider our allies (that we do not know) to be inferior. This is a "I, me, myself" mentality. We believe that our resources should be wholly to help ourselves, as they would be a waste on our allies who will not be able to use them. For this and similar reasons, we are not so inclined to help our allies defend a push, as we would rather concentrate on getting gold (DOTA players read - farming) in our own separate areas (lanes); its a "so long as my tower doesn't die, I can't be bothered if my ally dies" kind of train of thought.
This also leads to a form of mutual distrust, especially between two people who think the same way as described above. It also makes the team in general more inefficient. One example is in the concept of sharing items in a team. Generally, this is cost-efficient (for DOTA players, one example is sharing regen items e.g. RoR, RoH, sobi masks between teammates in the same lane); however, in public games, no one is going to do this - they are all scared of "what if he takes my item and runs away with it?"
The second factor in pubbies is the idea (for most people) that no one is going to remember you after this game viz. the anonymity hidden behind your nickname or handle. As a result, firstly, you won't play your best - so what if I lose or play badly? You may not think so, but subconsciously you will excuse yourself even if you lose; unlike an inhouse, where the losing team thinks long and hard about why they lost and may even do a detailed analysis. Secondly, you are even free to insult and even malign your enemies (and allies!), and even threaten them with physical harm (the most common line goes something like "Where do you live? I'll come over and beat you up") ; all will be forgotten after the game. Thirdly, and this is related to a point above, you will not be so eager to help your team out - personal gain comes first - as no one will remember your 133t teamwork.
One common complaint players make in pubbies is that "My allies are so noob! They never help out or stick together!" In many cases, it is not solely the individual skill of the players that cause such complaints (although in some cases, that is true indeed) - it is the subconscious mindset described above that causes such behaviour.