[S.T.D.]

Agreed, but with all their knowledge, they were willing to live under the guidance of their elders ( FIL, MIL both ) and the wife didnt try to force her husband to move to another house.

I didnt mean to imply that it was so, only that it could have been one of the causes. But in the present day scenario, isnt the feud between the Mother-in-law and Daughter-in-law a primary factor for the sons to set up a separate establishment ? Of course, they give enough explanations for doing so MIL : "my son's office is nearer from there"; or the DIL : "my daughter's school is quite distant from my in law's place", and so on ! We seldom see any instance of the FIL and SIL being the cause for it.

I am not so sure about this ramky. If you read the story of Kanaki and Kovalan they lived in a separate house even when they were living in the same town/village as their parents. I don't think Kanaki would have urged Kovalan to set up a separate household. I am of the opinion that Joint family system started for mere convince than for any thing else and stopped for the same reason.
The whole MIL-DIL thing is a hype and its taken to new levels of exaggeration by all these tele-serials.
Its difficult for 2 different people to get along well. In the case of Husband and wife love between them plays a major role. But the same thing cannot be applied to other relations including mother,father,brother,sister,-in-laws. So there will always be friction and this is blown out of proportions and is made to look larger than life by jokes,stories and serials.

Well I have seen such instances in Gujarati, Marwadi, Punjabi & Sindhi families but dont know reg Tamil families. ....

There are a few Tamil joint families and as i said before, they are all trading families mostly hotliers and farmers.