Have you had your thyroid checked? Sounds like a symptom of hyperthyroid or just really good metabolism. A blood panel wouldn't be a bad idea to rule out any metabolic disorders. Next time you visit your doctor ask him to do a metabolic panel on you to check your TSH levels for hyperthyroidism and any other metabolic disorders.

Eat a pound at least of meat per day plus at least 200g carbs (doing a lot of one or the other doesn't work) and diligently perform whatever weightlifting routine you think you can stick with. This is how I finally started to gain some mass. I'll race ya to a 50lb gain.

Does not have to be simply genetic, and I would seriously doubt a thyroid issue. Quite possible you programmed yourself early on to be running at this rate and are just fortunate enough not to be horribly effected by the SAD of your past (yet, 31 really isn't all that old :).

General activity level does a lot more than many give it credit for. Just living life instead of resting your rump counts for a lot. There is no reason you should be gaining 1lb a year anyhow unless you are actually trying to increase lean mass. Exercising and increased calories will do that to an extent.

Not really paleo by definition but GOMAD (Gallon Of Milk A Day) is touted as an easy way to gain weight. The idea that you drink an extra gallon of milk a day on top of your normal foods. This you will find it easily googleable online, just one option you can look at. Might be slightly better than going SAD and spiking your insulin(fat store hormone) levels with white bread and rice, than gorging a ton of food like sumos do

I'll just share my experience here because I'm at the same point as you right now. I started Paleo just over 15 months ago because I couldn't loose weight and I was 30lbs over weight and also very sick from food allergies.

Finally about 3 months in started to shed the weight and really melting the fat off, about 10 months into it I was at my 'normal' weight but then suddenly I couldn't stop shedding the weight and I was getting pretty gangly looking. Although I felt great in general.

I started loosing too much so I had no choice but to start adding in things like potatoes, small amts of sweet potatoes, occassionally white rice, found I could tolerate occassional quiona and quiona pasta. This helped and now I'm holding (took me about 2 months). Oh, I did find a almost pure haagendaz vanilla ice cream and had about a 1/4 c every 2nd day. (I'm not off dairy completely, and I've also added in home fermented sour cream and use some heavy cream in sauces and use ghee)

I have problems handling large amts of fat so adding in coconut milk/cream/flakes/fat, avocados could only be tolerated in small amounts and in rotation. I also couldn't get enough calories to make a difference, I seemed to finally be a metabolic burning machine, and this without any exercise other than walking the dog 3-4 times a day around the tiny park by my apartment.

I don't have thyroid issues, I do have issues staying hydrated in general, but other than some occassional IBS and food allergies (which I avoid eating those things), I am of decent health.

So perhaps if you are well in general try adding in some of the more 'starchier' foods over a few days (look for things that are dense so you won't need to eat large portions), one at a time and see if they agree, then start looking for more. Over time perhaps this will help. This worked for me, but everyone is different!!

I've been a hard-gainer my whole life as well. and friends would always be amazed at how I couldn't gain weight despite the quantities (and types) of food I ate. Paleo didn't help, as I actually shed a few pounds when I started.

The only thing that has helped in the last year is doing a weight-lifting program like Stronglifts or Starting Strength. When I stick to the protocol, I do eat more than I did before, and gain weight accordingly. When I slack on the program, so does my weight gain.