I genuinely used to think I was decent at multitasking. Emails during meetings, texting while cooking dinner, half-watching something while writing. Felt productive at the time. Then I stumbled on a study that actually measured this stuff, and turns out multitasking lowers IQ by a real, testable amount. Not some vague “you seem scattered” observation. An actual number. And it was bigger than I expected, honestly.
This isn’t really about being clumsy at juggling things. It’s about what’s happening inside your head every time you jump from one thing to another, and why that jump costs way more than it feels like in the moment.
The Study That Showed Multitasking Lowers IQ
Back in 2005, a guy named Glenn Wilson at the Institute of Psychiatry, King’s College London, ran a test that’s still getting cited constantly today. Two groups sat down for an IQ test. One group just took it, no distractions. The other had to keep checking emails and replying to instant messages while doing the same test. Average score for the distracted group dropped by 10 points. For comparison, that’s a bigger hit than what shows up after a rough night with zero sleep, and roughly double what researchers see after someone’s smoked weed. Ten points is a lot, actually. Enough to knock someone from an average score down into a noticeably lower range, just from trying to do two things simultaneously.
What stuck with researchers wasn’t only the number though. It’s that the effect showed up even with mild distraction. Just knowing messages were sitting there unread was enough to chip away at performance. Didn’t even need to be actively typing a reply.
Why Task-Switching Isn’t Really Multitasking at All
Here’s the part that got me. Your brain isn’t doing two things at once when you “multitask.” It’s switching back and forth, fast, and every switch has a cost attached, a cognitive switching cost. Jump from writing an email to reading a text and back, and your brain has to drop one set of rules and reload another. That reload takes real time and real mental energy, even if it feels instant while it’s happening.
Do that dozens of times an hour, which honestly is just a normal day for most people glued to their phone, and those little costs stack up fast. That’s part of why a day spent “multitasking” often leaves you more wiped out than a day spent grinding through one hard task start to finish. Mental bandwidth isn’t some bottomless well. Constant switching drains it quicker than sticking with one thing does.
I looked at a closely related angle of this while digging into why app notifications are basically engineered to grab your attention at the worst possible second. Not an accident, by design. How notifications are designed to interrupt you at the worst time gets into that mechanism more if you want the full breakdown.
Divided Attention and Why Your Brain Can’t Actually Split Focus
People misunderstand divided attention a lot. They picture it like a pie sliced in half, focus split evenly between two things. That’s not really what’s going on though. Attention works more like one spotlight jumping fast between targets, not two spotlights lighting up at once. So when you’re “multitasking,” that spotlight is just flicking back and forth quick, and each flick leaves behind a little residue from whatever you were just doing. Researchers call it attention residue.
That leftover residue is a big chunk of why multitasking lowers IQ in these controlled tests. Part of your working memory stays busy processing the last task even after you’ve technically moved on, leaving less room for whatever’s in front of you right now.
Stanford ran a separate study looking at people who constantly multitask with media, texting, browsing, watching, all stacked together, compared against people who barely do it. The Stanford study on cognitive control in media multitaskers found heavy multitaskers were actually worse at filtering out irrelevant stuff, not better. Which goes against the whole idea that constant multitasking somehow trains your brain to get good at it. Stanford’s own writeup on the research said heavy multitaskers were unusually easy to distract, even when they were trying hard to focus on just one thing.
The Stress Layer Hiding Behind All This Switching
There’s a stress piece tangled up in here too, making it worse than it already is. Bouncing between tasks keeps a low simmer of urgency running the whole time, kind of similar to the stress response that quietly wrecks memory formation over the long run. I got into that connection while researching how chronic stress messes with memory consolidation, since these two problems tend to show up together in people who are juggling too much. Stress and memory loss covers how the two compound each other if that’s interesting.
Same pattern shows up in weirder places too, like meetings scheduled right after lunch. Natural energy dip collides with mental fatigue from a morning of constant switching, and focus takes a real hit. Why meetings after lunch are hard to focus in digs into that timing angle if you’re curious.
What Actually Helps
None of this means you have to cut multitasking out completely. Just means being more intentional about when it happens. Group similar tasks together instead of hopping between totally unrelated ones. Kill notifications during anything that actually needs real focus. And give yourself a couple minutes of buffer between tasks instead of jumping straight from one to the next, since that little gap helps clear out whatever residue’s left from the last thing.
FAQs
How much does multitasking actually lower IQ?
Research out of King’s College London found roughly a 10-point drop during multitasking conditions. Bigger temporary hit than researchers usually see after a bad night’s sleep.
Is multitasking lowering my IQ permanently?
No. The drop measured in studies is temporary, tied to the moment you’re actively switching, not some lasting hit to your intelligence.
Why does switching tasks feel more tiring than just focusing on one?
Every switch has a cognitive switching cost attached. Your brain drops one task’s context and reloads another, and doing that over and over drains mental bandwidth quicker than staying focused does.
Does multitasking get easier the more you do it?
Not really, based on the research. Heavy media multitaskers actually did worse at filtering distractions than people who rarely multitask. Practice didn’t seem to build any real immunity.
What’s a simple way to cut down the cognitive cost of multitasking?
Batch similar tasks, silence notifications while doing focused work, and leave a short gap between tasks so leftover attention residue has time to clear out.
Final Thoughts
The research is pretty clear on this one. Multitasking lowers IQ in a way that’s measurable, temporary, and tied straight to how much your brain’s switching rather than actually splitting focus. Once you see task-switching for what it really is, quick, expensive jumps instead of true parallel thinking, it gets a lot easier to protect whatever mental bandwidth you’ve got left.