This is really long, sorry, I've tried to explain though about my own depression because I think your question is very important but answering it is so loaded and complex, so I've tried to cover the bases. I'm very interested to read others opinions on this.
I have clinical depression, was diagnosed in my early teens and was given prozac at 13 (I'm 30s now, still on the sunny Ds, just a different kind) but I doubt it my life had been less horrific than it was I'd have perhaps not had clinical and just shit life syndrome, however because of my bizarre upbringing I don't think my brain is wired properly which is why I have depression now even though my life is, besides being a skint from time to time, fucking peachy!
I have a irritatingly positive outlook etc but I can still find things to wind myself up over to the point I struggle getting out of bed.
I am poorly educated, no SAT'S let alone GCSEs but I'd have perhaps been quite academic in other circumstances. I am what was known as street smart which just is a fancier term for neglected estate kid in my case.
I do think that depression can be caused by feeling sadness from others.
Say those parents on press conferences of missing children, seeing the news of the Palestine/Israel drama, I think depression can be secondary almost.
So your life can be as rosy as Kate Winslets cheers in titanic and if you're someone who feels hurt on the back of world injustice or sadness, depression will worm its way into your mind and make itself very comfortable, like a squatter with an oodie. And be just as hard to shift.
I think people who are susceptible to feeling pain of others, regardless of their academic abilities are the ones more likely to experience depression.
Because even when things are going great for you, it's hard to witness the world around and it not cause despair.
I know for myself personally and I speak only for me here, but I can be having the most wonderful day, the things that fill me with joy can be surrounding me and yet I'll see something that is categorically not my fault, my problem or my business, eg homeless person and I can't shake the guilt and sadness then that they're living such a poor life when mine is going swell.
It can lead to manic episodes then, so I'll have to buy homeless people selection boxes and hot drinks and I can spend my last ten quid doing it and it won't help, I'll just feel despair that I haven't done enough. So often my depression can just spiral. I never ever let anyone outside see that, I have it well hidden, I'm as present as a mum, partner, friend I can be, but inside it takes so little to send me into crushing sadness and I know logically that it's an illness, that probably the homeless wouldn't want me crying in bed about them (I've been homeless so it's a massive trigger) but it's harder to pull myself out of the mindset than I admit.
I'm not really bright, not really dim, so I'm not sure if it makes a difference, I agree with what CurlewKate said. But I'm still not sure if ultimately intelligence is a factor, but I think you'll be judged on your depression with your social economic status.