Turns out, a lack of friends can shorten the human lifespan as much as more well-known killers like smoking and alcohol consumption. In fact, the effects of social isolation actually outweigh some of the much-touted risk factors for mortality, like obesity and lack of exercise. In other words, you might be better off sitting on the sofa at home rather than going jogging, as long as there's someone else on the sofa with you.

Earlier studies blamed this early-death phenomenon on loneliness: after all, being lonely sucks, and there's only so many birthdays you can spend crying alone in front of a single cupcake with a candle in it before it starts to affect your health. But more recent research suggests that it's the isolation itself that does the killing. A 2013 study of elderly people in the U.K. found that while simply reporting feelings of loneliness had no effect on one's likelihood of dying anytime soon, social isolation increased a person's likelihood of death by 26 percent when compared with more social people in a similar age range. So even if you're perfectly happy to spend all your time alone accompanied only by a vaguely human-shaped mannequin painted to look like Andrew WK (and who wouldn't?), Death's bony form will still be creeping up behind you.

How Does That Work?

Part of this is obvious: if you avoid other humans, you're less likely to figure out that the lump on the back of your neck is growing, or that one of your limbs has become possessed and needs to be removed. And no matter how much your cat loves you, it probably won't call an ambulance when you're choking.

Martin Poole/Digital Vision/Getty Images
It might put together a nice bouquet for the funeral, though. And then eat it.

But there are also physical causes behind humanity's propensity for solitude-death. Physical contact between humans has been shown to reduce stress and inflammation, so isolating yourself from humanity is a good way to make your health go south. Not to mention the fact that most of us just tend to take better care of ourselves when others are around: having other humans in the house often inspires a kind of behavior-improving shame that causes us to, say, get a healthy eight hours of sleep instead of staying up all night trying to make deep-fried s'mores in our bathtub. And if the thought of dying alone and sad is getting you down, you should try to cheer up, because another thing that will kill you is ...

How Does That Work?

It's no big secret that people with depression are at higher risk for suicide, as well as for less-than-healthy activities like drinking too much. But this sad rabbit hole may go deeper than that. Recently, scientists have been looking at the effect depression has on your body at a cellular level. They do this by measuring people's telomeres, which sounds kind of dirty but actually just refers to a part of your chromosomes that shortens with cell age. Looking at the telomeres within a cell can tell you how old and worn-out that cell is feeling. The shorter your telomeres are, the more you're at risk of an early death from age-related ailments like heart disease and cancer.

So researchers looked at blood samples from three groups of people: those who had never been depressed, those who'd had major depressive disorder in the past, and those that were depressed right at that moment. After ruling out lifestyle factors like drinking and smoking, they measured the subjects' telomere length and discovered that the never-depressed group had longer telomeres. What's even creepier is that the more severe the depression the subjects reported, the shorter and more age-related-disease-prone their telomeres got. It's as if your body can measure the depths of your existential despair at a cellular level, like some really fucked-up version of that pain scale they give you in hospitals.

Via CDR AMC
There's no Vicodin that can cover the hurt in my soul.

The blame here seems to lie with the effect of major depression on the immune system, as well as that old bastard that is always ruining everything, inflammation. So the more depressed you are, the worse it is for your health, and holy Christ that realization is really depressing, and- oh God I've just made it worse, haven't I.

How Does That Work?

Gum disease, or infection of the soft tissue around the teeth, affects about half of all adults in the United States, because apparently none of us listen to puppets. And we've known for a long time that there is a link between gum disease and heart disease, also known as this country's most prolific murderer. The question has always been whether that link is actually causal, or if it's just that the activities that put you at risk of gum disease (like poor diet) also happen to be bad for your heart.

Jack Hollingsworth/Photodisc/Getty Images
Both conditions have been going up since we replaced tap water with gummy bears.

But now there's at least some evidence that neglecting your gums can fuck up your heart directly. In an experiment reported earlier this year, researchers infected mice with several types of bacteria that cause gum disease, and then tracked the spread of the bacteria until it reached their hearts. The mice then started showing an increase in risk factors for heart disease, like inflammation and cholesterol. These mice weren't doing anything else that put them at risk for heart disease, like, I don't know, smoking little mice cigarettes or working too hard at their high-stress mice jobs. This suggests that gum-disease bacteria alone might be enough to mess with your heart. So, by all means, start flossing, because it's pretty much the same as strangling the Grim Reaper to death.

And it's not just a longer life. Fall babies seem to do better overall: babies born in the spring are more likely to suffer from eating disorders, Type 1 diabetes, and schizophrenia. So if you're pregnant and reading this in Australia, disregard the good news from earlier and try to keep your legs closed for another six months or so.

How Does That Work?

Nobody knows for sure why fall babies seem to have it better, but one theory is levels of Vitamin D during pregnancy. People usually get their Vitamin D via sunlight, which means that by the time they give birth, mothers of fall babies have been basking in Vitamin-D goodness for the last six months or so. Spring babies who do the majority of their gestating during the darker months are more likely to have mothers who are low on sun juice, and this might somehow affect their development and long-term health.

Via Time.com
Or it could be caused by the anger of an Aztec sun god. I'm keeping my mind open.

Another possible factor is diet. The people in the "lived to 100" study were mostly born in the 1890s, when stuff like refrigeration and food transport wasn't exactly up to today's standards. It makes sense that babies who gestated during the warmer months, when fruits and vegetables were more easily available, might do better than babies whose mothers lived on a more limited winter diet. So if you're one of those people who survives on ramen during the winter because it's too much effort to go out in the cold and buy groceries, think again, or your descendants might die young.

C. Coville's book, One-Star Reviews, is available for pre-order on Amazon right the hell now. She also has a Twitter here and a Tumblr here.

For more from C. Coville, check out 4 Everyday Things That Make Us Way Angrier Than They Should and 5 Well-Intentioned Phrases That Have Been Ruined by Assholes.