How do I unlock higher difficulty levels in Diablo III?

I know that there are four difficulty levels in Diablo 3 as follows:

• Normal
• Nightmare
• Hell
• Inferno

Can the difficulty level be changed manually or does it automatically increase on subsequent play-throughs (i.e. moving from normal to nightmare, nightmare to hell) as in Diablo 2?

Solution 1:

The 4-tier difficulty level is tied to progression throughout the game. To unlock "Nightmare" (the 2nd difficulty), you must first complete act IV on "Normal" difficulty. Nightmare is balanced based on the level you would be by beating through Normal, so it's not feasible to just skip ahead, even if you could.

The other factor is Hardcore, which is unlocked at level 10. Hardcore is, well, harder, and is separate from the game difficulty presented elsewhere. You want more of a challenge? Play Hardcore and try not to die!

Solution 2:

You have to beat them in order to unlock the next one. There is no way to skip around. This goes for every character you make as well.