"People are mean."
I've read that the plural marker 들 is optional. You can use it to emphasize the fact that it is plural, but you can also leave it out. (But OMG, I know so little -- so please wait for others' comments. ...)
Oh, I actually seem to be right; there's a similar discussion: https://www.duolingo.com/comment/24421391
들 - Is use to emphasize that the noun is plural
If a noun does not have 들 , in can mean singular or plural .
If you want to emphasize that it is singular , for example , There are just 1 Person ' , You can use : 한 (One) 명 ( Use to indicate number of people ) 사람 ( Person/People )
한 명 사람 있어