Which Factors Increase or Reduce the Risk of Depression?
What 100,000 people taught scientists about preventing depression and the one habit that beat them all.
6/8/20265 min read
Imagine two people with the exact same genetic risk for depression. One spends evenings confiding in a close friend. The other unwinds with hours of television. Years later, researchers checking in on both would find something striking: the friendship wasn't just nice to have. It may have actively lowered the risk of depression, even in someone genetically wired to be vulnerable.
That's the heart of a remarkable study from Massachusetts General Hospital and Harvard, published in the American Journal of Psychiatry. Depression is the leading cause of disability on the planet, yet for decades we've had surprisingly few proven ways to prevent it. Instead of testing one or two hunches, the research team screened 106 everyday habits and life circumstances in more than 100,000 people to find out which ones genuinely move the needle. They even used DNA to check whether those habits actually cause depression, or just happen to travel alongside it.
Why Most "Depression Advice" Doesn't Hold Up
Here's the uncomfortable truth behind a lot of wellness headlines: just because something is linked to depression doesn't mean it causes it. This is the classic correlation-versus-causation trap.
Take a simple example. Depressed people may spend more time on the computer, but does the computer cause the depression, or does the low mood lead to more screen time? Observational studies alone often can't tell the difference. They can be fooled by reverse causation (the illness driving the behavior) or by hidden third factors lurking in the background.
This is exactly why so many "miracle" lifestyle fixes fizzle out when put to a rigorous test. The researchers wanted to separate the real prevention targets from the noise.
The Two-Step Method, Explained Simply
The team used a two-stage approach, and it's worth understanding because it's what makes their findings trustworthy.
Step one: They tracked over 100,000 UK Biobank participants who had no significant depression symptoms at the start. They measured 106 modifiable factors, including exercise, sleep, diet, social life, screen time, even air pollution and green space. They then checked back 6 to 8 years later to see who had developed clinically significant depression.
Step two: For the habits that looked important, they ran something called Mendelian randomization. Think of it as nature's own randomized experiment. We're each dealt a random genetic hand at birth, including genes that nudge us toward certain behaviors, like being more socially open or more prone to napping. Because these genes are essentially randomized at conception and fixed for life, they can't be "caused" by depression later on. So if a gene-driven tendency toward a habit also predicts depression, that's powerful evidence the habit truly causes the change, not the other way around.
It's the difference between noticing that umbrellas and rain appear together, versus proving which one actually makes the other happen.
The Habits That Survived the Test
Out of 106 factors, 49 were initially linked to depression. After accounting for income, education, and physical health, 29 still stood. But only a select few passed the rigorous DNA-based causation test. Here are the standouts.
1. Confiding in Others — The Clear Champion
The single most powerful protective factor was frequency of confiding in others. People who regularly opened up to someone they trusted had significantly lower odds of developing depression.
The numbers are compelling. In the observational data, confiding in others showed the strongest statistical signal of any factor. And when put through the genetic causation test, it held firm: an odds ratio of 0.76 — meaning higher levels of confiding were associated with roughly a 24% reduction in the odds of depression. Crucially, this protection showed up even among people at high genetic risk and among those who had survived traumatic life events.
This wasn't about having a thousand acquaintances. It was about having someone, even one person, you can genuinely talk to.
2. Television Time — The Quiet Risk
On the flip side, television watching time emerged as a genuine risk factor, confirmed by the genetic analysis (odds ratio of 1.09). More TV time was causally linked to higher depression risk.
The researchers are honest about a nuance here: we don't yet know if it's the screen itself, the content, or simply that TV is a stand-in for sedentary behavior. Either way, the signal was real and held up even among at-risk groups.
3. Daytime Napping — A Two-Way Street
Here's a surprising one. Daytime napping in adults was linked to higher odds of depression (odds ratio of 1.34, one of the largest effects in the study). But the genetic analysis revealed the relationship runs both ways: a tendency to nap appeared to raise depression risk, and depression itself appeared to increase napping.
This doesn't mean a refreshing power nap is dangerous. It suggests that a pattern of frequent daytime napping in adults can be both a warning sign and a possible contributor worth paying attention to.
The Surprises and the Letdowns
Science is at its most honest when it overturns our assumptions, and this study had a few.
Multivitamins didn't help. Despite their feel-good reputation, multivitamin use was actually linked to higher depression odds, though this finding weakened under stricter testing and likely reflects reverse causation (people feeling low may reach for supplements). The same went for vitamin B supplements, which showed no protective benefit, consistent with the lack of solid clinical trial evidence.
Exercise and sleep are complicated. Physical activities like swimming and cycling looked strongly protective in the observational data, but the genetic test couldn't fully confirm a causal effect, likely because self-reported exercise is hard to measure genetically. Sleep duration showed a similar story. This doesn't mean exercise and sleep don't matter. Earlier research using objective trackers has confirmed exercise's protective power. It means self-reported versions of these habits are harder to validate with this particular method.
Air pollution and green space faded. Environmental factors lost their signal once income and neighborhood were accounted for, hinting their effects may matter more in childhood or depend on specifics like tree cover versus open grass.
The Brain-and-Body Connection
Why would talking to a friend ripple all the way down to your mental health? Think of social connection as a buffer system, much like a car's suspension absorbing shocks in the road. Stress, setbacks, and even genetic vulnerability are the bumps. A trusted confidant helps absorb the impact before it rattles the whole system.
The researchers note this fits a growing body of work, including a study of military personnel where strong social cohesion protected against depression despite high genetic or environmental risk. Connection appears to be one of the few levers powerful enough to push back against biology itself.
What This Means for You — Practical Takeaways
This study isn't a prescription, and the authors are careful to note that lasting proof will require clinical trials. But the genetically validated findings point to genuinely actionable steps:
Build a confiding habit, not just a social calendar. The protective factor wasn't how many people you know, it was having someone you can actually open up to.
Audit your TV time. You don't have to quit cold turkey. Try swapping one viewing block for a walk, a call, or any activity that gets you moving and connecting.
Notice your napping patterns. Frequent daytime napping can be an early flag worth discussing with a doctor, both as a possible contributor and a signal of how you're really feeling.
Don't lean on supplements for your mood. The evidence here offers no support for multivitamins or vitamin B as depression preventers. Save your money for things with stronger backing.
Keep moving. Even though self-reported exercise didn't fully validate here, the broader science strongly supports physical activity. This study simply couldn't confirm it with genetics, not a reason to stop.
The bigger message is hopeful: even if depression runs in your family or you've lived through hard times, your daily choices (especially around connection) may still tilt the odds in your favor.
Who in your life could you confide in this week?
Source: Choi KW, Stein MB, Nishimi KM, et al. "An Exposure-Wide and Mendelian Randomization Approach to Identifying Modifiable Factors for the Prevention of Depression." American Journal of Psychiatry, 2020; 177:944–954. doi: 10.1176/appi.ajp.2020.19111158
This article summarizes research findings and is not medical advice. If you're struggling with depression, please reach out to a qualified mental health professional.
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