'Me,' said Ron, who was still rather dishevelled. 'I didn't realise that would happen. It's not fair!' he added to Harry, as the girls headed off for the portrait hole, still giggling madly. 'Hermione's allowed in our dormitory, how come we're not allowed -?'
'Well, it's an old-fashioned rule,' said Hermione, who had just slid neatly on to a rug in front of them and was now getting to her feet, 'but it says in Hogwarts: A History, that the founders thought boys were less trustworthy than girls.
-Harry Potter and the Order of the Phoenix chapter 17
Also,
- Was the dormitory spell cast by Gryffindor only or was it present in all dormitories of all houses?
- Are male house-elves like Dobby allowed into girl's dormitories? If not, then who cleaned their dormitories during Hermione's sock revolt?