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Instagram is full of seemingly happy plant owners cultivating succulents and philodendrons. You might have even seen local plant stores advertise their wares as stress-reducing roommates.

And the truth is, there is some evidence that houseplants are good for your mental health. It's just not really clear why. We've been studying how houseplants make us feel for decades in a range of different indoor situations, but some of the most robust evidence that houseplants can improve mental health comes from hospitals.

For example, a study from 2009 looked at how the presence of plants and flowers affected the recovery of 90 patients in the hospital for hemorrhoidectomies — and that is what it sounds like. It's a time in your life when it's probably not bad to have a plant friend around. They found that patients in rooms with plants did fare better: their blood pressure was lower and they reported less pain, anxiety, and fatigue.

The patients also said that the plants helped them to relax and feel less anxious, and made them feel more positively about the hospital staff caring for them. And the researchers who led this study have found similar results with plants in the rooms of appendectomy and thyroidectomy patients as well. The thing is, these hospital studies don't tell us a whole lot about why this effect occurs—just that it seems to.

Researchers have tried to drill deeper into mechanisms by looking at plants in other indoor environments, like offices. And these studies seem to back up the idea that plants can positively affect our moods, but the data start to get a little bit murkier. One of the big questions is whether plants affect specific cognitive processes, and therefore make us feel better by lifting our mental load, or are just generally de-stressing.

Some psychologists think that indoor plants and other living things have a restorative effect, based on the idea of Attention Restoration Theory. According to this theory, voluntary attention—the attention needed for things that aren't inherently fascinating, but must be focused on nonetheless—is finite. But, if you can just stop focusing so hard on things, you can give your mind time to replenish its attention reserves.

And plants may help you do that because they are inherently interesting— so you don't have to force yourself to pay attention to them. One research team found that having plants in a room did help the 34 student participants to increase their attention capacity after a proofreading task. But, they felt that may have been because the plants made the room feel more pleasant and reduced the participants' stress levels in general rather than having a real restorative effect.

And there are good reasons to suspect that the presence of plants is just kind of relaxing. Like, in a 2013 study, 18 tested participants reported being more comfortable in an office environment with plants than a space without them. But even if a relaxing effect is real and directly induced by the plants themselves, it still doesn't tell us what about plants makes us feel less stressed.

Researchers in that 2013 study asked the participants to say what they preferred in terms of color, odor, and plant size, and found that the group favored small, green, lightly-scented plants. So it may be that those sensory experiences— seeing small bits of green or smelling living things—somehow triggers relaxation. But unfortunately, studies that have tried to tease out whether it's the color, three dimensional shape, scent, or some other aspect of a plant that really matters haven't shed a ton of light on things.

In part, that's because many of the studies use surveys to ask participants about their emotional and psychological responses to the plants. While these results are relatively consistent across studies, they're subjective measures prone to all kinds of biases. And, linking mental states to more objective measures like physiological changes is tricky at best.

Many studies look at changes to blood oxygenation in parts of the brain, for example. Participants in these studies wear a special sensor that uses spectroscopy to measure near-infrared light absorption; you see, red blood cells absorb light differently when they're carrying a lot of oxygen. And since oxygenation and blood flow in the brain have been linked to neural activity, this measure might give the researchers a peek at how the brain is responding to different conditions.

But studies often get confusing results. Like, in a 2016 study, researchers had 24 men in their 20s look at either live plants or an empty flower box for 3 minutes. After that, the scientists detected a decrease in blood oxygenation in the right prefrontal cortex in the plant viewing group.

That went hand-in-hand with reports of feeling more. Comfortable and relaxed by viewing the plants. So you might think less prefrontal cortex oxygenation equals a more relaxed state of mind.

But another study from 2015 found significant increases in blood oxygenation in both the left and the right prefrontal cortex when 18 participants viewed a real plant as opposed to a projected image of it. This lack of consistency shows just how difficult it can be to untangle the correlations between physiological and psychological responses. And some researchers have been critical of the methodology of studies looking at indoor plant effects on mood in general.

A 2009 review of 21 studies took issue with a number of methods in the field, including the fact that studies often used small sample sizes. You wouldn't expect the effects of indoor plants to be huge— and if they have small effects, from a statistics perspective, you'd need a large number of participants to pick them up. Similarly, they criticized the variation in how the plants were presented in different studies.

The type, size, and placement of plants, as well as duration of exposure all differ greatly—and that makes results difficult to compare. But the reviewers did feel that the question of whether or not indoor plants can affect our mental health is worth studying, so they made some recommendations for future experiments. These included implementing more systematic approaches to plant exposure and making sure that studies looking at the “restorative” effects of plants actually had the participants do something taxing enough to require restoration.

So… Even though we've been studying this for decades, we still don't seem to understand why we see such positive effects from plants in places like hospitals. Or if those mean houseplants boost your brainpower or improve your mental health in general. But look, here's the situation, those plants on your desk and by your couch are almost definitely not harmful to you, as long as you're not allergic to them or something.

So if you feel like your world is better when your desk or living room looks a little bit more like a jungle, you do you! Thanks for watching this episode of SciShow Psych, which is produced by Complexly! If you enjoyed it, you might also enjoy our podcast SciShow Tangents.

Several of the minds behind Complexly shows—including me— get together each week to nerd out about science in a playfully competitive way. Sam, who is sitting here making sure I don't say any words incorrectly, is currently winning, significantly, and we're getting a little bit more aggressive about trying to bring him down. But the real point is that we share all sorts of astounding facts about our universe, our planet, and our species.

You can listen for yourself by searching for SciShow Tangents on the podcast platform of your choice. [♪ OUTRO].