to settle an argument that social isolation has caused...

[guffaux]

underlying causes which no-one knew about

or

underlying causes that anyone knew about

Uhm... what?

They mean the opposite of each other dont they?

The first

Wouldn't you have to have 'no' at the start so then the first would be a double negative? Or something.

yes,
'no' comes at the beginning of the sentence...

If you’re starting with ‘no’ then it’s the second.
With no ‘no’ it still reads but in that instance it would be the first.

Either. But one needs a comma?

underlying causes, which no-one knew about

or

No underlying causes that anyone knew about

and technically a sentence or a clause shouldn't end with a preposition.

So:
Underlying causes, about which no-one knew,.......
or
No-one knew about (the) underlying causes.

Ah.. so sorry! And agree with wonderrotunda..

thankyou for your replies... helping the argument along nicely