Which is correct way to write a decade: the 1920s or the 1920's?

Question

Searching the web brings back about 50/50 results and I have consulted two different grammar books which are both different.

Answer 1

1920s.

Answer 2

1920s

Some people write it with the 's but it's a mistake.

Answer 3

both

In 1920s people could witness....

... thus the political situation of 1920's and 1930's led to the further...

Answer 4

1920s. you are welcome.

Answer 5

you write 1920s when youre referring to the era and when youre referring to the fact that the 1920s had say flappers or speakeasies you the 1920's flapper/ speakeasies. the ' is possesion.