I'm not talking about a late night writing a term paper; I'm talking about publishing work already published, and taking credit for it. There are numerous examples of people doing just that--even submitting it for an award! You would think a moment's thought would stop someone from sending a plagiarized work to a bunch of judges, the people most likely to recognize the source material, but no.
There are three reasons, as far as I can see:
1. One is a pathological liar
2. One is stupid or ignorant enough not to understand what plagiarism is
3. One is motivated to plagiarize more than you are not to.
The first stands on its own, and needs no explanation, I think. The second is most likely in those unable to publish anyhow, or because someone doesn't understand the difference between a "fair use" quotation and plagiarism. And many scholars and writers have certainly taken stories and rewritten them--perhaps not enough.
The third, on the other hand, may be something deeper.
Motives are powerful emotional drivers--what if some of them drive you in the wrong direction?
Some studies have indicated that when one has high Achievement motive and it has been strongly aroused by a situation--like a game or a contest--it is more likely that someone will "cheat" to get to the goal. This may account for the bad reputation that some kinds of salespeople have, since short-cycle salespeople tend to be driven best by Achievement motive. David Winter, who has studied American presidents extensively, has found that Achievement-motivated presidents tend to get caught more often in campaign finance irregularities, Nixon being a classic example of this. Think of it this way: if you need $20 million for your campaign (chicken feed, I know, but go with it), is it more efficient to get a 20 bucks each from a million people, or a million dollars each from twenty millionaires? We know how Nixon answered that question. Being more efficient is a concern of the Achievement motive. Power/Influence-motivated Presidents tend to come from grassroots movements, because they'd rather know they touched a million people, even just as little as 20 dollars' worth.
You can see where I'm going here.
Achievement-motivated plagiarists are being more efficient--it's faster to cheat than write it out on your own.
Power-motivated plagiarists may just want the attention they can't get any other way and are desperate to get.
I see two other necessary traits here:
1. A lack of empathy in that you can hurt someone else implicitly by stealing their work or indirectly their reputation. I suspect a lot of people can persuade themselves that stealing from someone like Stephen King (say) is harmless--that they are entitled by the differences between them: "King would never notice if he lost five bucks, but that means a lot to me." Sorry folks, it is still stealing!
2. A lack of awareness of implications: ever consider what will happen if you are found out?
Or it could be simple selfishness, which has elements of both. It is worth noting, however, that if the emotional drives are powerful enough, they can overwhelm good sense and reason and even empathy. Someone who is desperate can rationalize what they are doing.
One of the classic "moral dilemmas" used by Lawrence Kohlberg to study stages of moral development has to do with a man stealing an expensive drug to treat his ill wife, who may die without it. Is it justified? The way people grapple with this is more important than the answer they come up with--how do they reason?
But moral reasoning is still reasoning -- it's conscious thought. And this can fall by the wayside when you are emotionally aroused. There's a reason they talk about someone being "desperate" as being dangerous.
"I'm hiding in Honduras...I'm a desperate man.
Send Lawyers, Guns, and Money--the s*** has hit the fan."
--Warren Zevon, "Lawyers, Guns and Money"
Now, we're talking about writing, not life-or-death, right? Well, the funny thing about motivation is that it is unique to the individual. Things that make no sense to most people are overwhelmingly important to others.
"Man is not a rational animal--he is a rationalizing animal." --Robert A. Heinlein
And hence comes unethical behavior...