Forensic Psychology graduate here.
From what I remember it's rather basic, essentially if you're not physically punished (which happens but very rarely I think) then the 'bad cop' can say things like 'if you plead guilty and admit it you'll only get X years, but if you plead innocent then when the jury finds you guilty you'll get X+5 years, your family will have to cope without you, its a dangerous world where money is hard to come by' etc. Its not as black and white and its not like the movies where the bent cops threaten to 'take care' of the suspect's families, but its along those lines.
Also, exhaustion. After being given late night interviews and a disrupted sleeping pattern for so long many peoples brains will enter survival mode and do anything just to get rid of the environment, which includes confessing. The brain doesn't see the long term effects of imprisonment etc, but rather only sees a way out of the psychological and physical exhaustion.
If I'm wrong or there's more please add and tell me, but that's the gist of it from what I can remember.
Also note that from the studies I saw, what I mentioned above is the exact reason torture doesn't work, people will say anything to stop the pain.
The threat of being charged with a much more serious crime if they don't confess to a more minor crime.
Imagine you were accused of a crime you didn't commit, say armed robbery. Someone got shot but not killed. You're facing 30 years behind bars for this.
Say the prosecution offers you a deal to prevent this from going to trial and taking years - plead guilty to robbery and a weapons offense, get out in 5 years, maybe 3 with good behavior. Or, go to trial and risk being put away for 30.
Give up 30 years of your life, your entire life is over. Get out in 3, it's going to be hell but you only gave up 3 years of your life, maybe you can still reconnect with friends and family afterwards. Do you roll the dice with your entire life if they've got enough evidence to take things to trial?
You're locked in a room full other assumed criminals, removed from contact with the outside world, thrown into a disgusting place, sometimes beat, Fed food that can range from bland to green bologna. Your dignity is taken away, your respect, and your trust in the justice system. They tell you that if you just confess then you'll get to go home. You won't have to eat eggs that include the egg shells, at home you'll have that blood pressure medicine that you bet your life on and most importantly, 5 years on probation is easy, just sign the paper and we let you go.
Or to put it simple: sign for 5 years now or get life once the trial is over.
In addition to what others have posted, a very common scenario:
Considering how backed up our court system is there are innocent people in jail for years awaiting trial. At some point a prosecutor tells them that if they plead guilty to X their sentence will be time served. If they say no they risk staying in jail for an indeterminate amount of time and still possibly losing the trial.
Often the consequences of not confessing are worse than the consequences of falsely confessing to a minor crime. At least in the American court system.
For example if you get picked up at a DWI checkpoint and accused of DWI and you can't make bail you can sit in jail for 2-3 years before your court date and there's still a very good chance of you getting convicted. If you just fasley confess and take a plea deal you'll be out in a week (and won't be able to drive for a year or two).
So the consequences of fighting false charges would be 2-3 years in jail waiting for a court date and the consequences of falsely confessing are only a year or two license suspension.
This happens all the time with shoplifting, petty assault, domestic violence, failure to pay a traffic ticket, speeding, resisting arrest, most drug charges.
Sometimes it's a good idea to confess even if you didn't do it.
Trickery and coercion, sometimes both.
Beyond the obvious physical coercion, the police might say you are facing the death penalty, but if you admit you are dealing drugs instead, they can offer you a deal. The whole process of plea bargaining as been challenged (unsuccessfully) as being inherently coercive, and some can still be rejected on those grounds.
Other times it is just manipulation and fatigue. They take turns guilty tripping you for hours at a time, convincing you that you are "responsible", even if you didn't actually commit a crime. You get so tired and emotionally agitated, you will say anything just to make it stop.
These are excellent reasons to *always* assert you right to remain silent and ask for an attorney if you are arrested.
You ever have a significant other or a child or a sibling who wants something from you and they pester you over and over and will not let up until you finally relent, even knowing full well that by relenting you're doing something wrong? That's how it happens. Police get you in a room, suggesting you are guilty over and over for 12, 18, 20 hours at a time. You get tired, you're intimidated, and eventually you tell them what they want to let it end. They may even suggest that you'll be found guilty either way and by confessing you are making it easier on yourself even if you didn't commit the crime.
Where the hell do you people live? Don't you have rules about interviewing suspects? Doesn't anybody in your respective countries actually give a shit about convicting the real offender by, I don't know, using evidence to support a confession?
say you wanna get someone to confess to a crime, did they do it? who cares, a plea agreement is a successful case under your belt. so step one: find every possible charge you can think of, doesn't matter if it has a chance in hell of sticking, just pile them on, key here is make the person think they are looking at 130 life sentences for flicking their cigarette butt out the window or whatever you arrested em for, or make them think they are looking at a $300,000 fine or what have you, then play up any evidence you might have, no matter how circumstantial, play up how expensive it will be to fight it, how unlikely they are to succeed, make em really sweat...
then, offer them a lifeline, just confess, accept a plea deal for community service or a fine that's tailored to be less than it would cost them to fight it in court...boom! you got a confession.
There's (from what I've seen so far) a decent netflix special on this, "The Confession Tapes" Recommend a watch.
The usual technique is to chain them to a table and take turns yelling at them and saying how screwed they are. Feed them as little as possible and prevent them from rest or sleep for 36 hours or more so they are so exhausted they start to hallucinate.
Then get someone attractive they have not seen before run everyone else out of the room, sympathize with the suspect, and give them a gift such as a sugary drink. Have that person promise that if they just sign the confession they will take care of the suspect and make sure everything goes away. They promise they will help and the only way they can get the suspect out of the situation is the confession.
Then they sign because they are barely coherent, or they don't and you sign them up for another 24 hours of sleep deprivation and abuse before the offer comes again.