The Actual Science Behind Why We Overthink

Have you ever replayed a conversation from three days ago. Or analyzed a slightly-off text for the fourteenth time. Do you lie in bed running the same scenario on a loop, even though you already know how it ends? If you're AuDHD, this probably isn't rare — it's practically a background hum. It happens to me daily.

It's easy to call this "overthinking" and leave it at a personality trait, or worse, a character flaw. But there's a specific, well-studied brain network behind it, and understanding how it works changes the story from "I can't stop doing this to myself" to "this is what a specific network in my brain is currently doing, and here's why."

Let's break it down so you can better understand the "why" behind overthinking...

Meet the Default Mode Network

Every brain has a set of regions that activate when you're not focused on an external task — daydreaming, remembering the past, imagining the future, thinking about yourself or other people. This is called the Default Mode Network, or DMN. It's the network active when you're staring out a car window, standing in the shower, or lying awake at 1am.

In a typically functioning system, the DMN and your task-focused attention networks work like a seesaw: when you need to concentrate, the DMN quiets down so attention networks can take over; when you're at rest, the DMN switches back on. That handoff is supposed to be smooth and automatic.

Where it goes differently in ADHD

Research shows people with ADHD have atypical connectivity in the default mode network — and one leading hypothesis is that increased activity in this network may interfere with the function of networks involved in attention and cognitive control. Instead of the clean handoff described above, the DMN in an ADHD brain doesn't fully quiet down when it's supposed to. It keeps competing for bandwidth, even during tasks that need focus — which helps explain both the mind-wandering and the racing, looping thoughts that show up at the worst possible times.

The specific link to rumination

This is where it gets directly relevant to overthinking specifically, rather than distraction in general. Neuroimaging research has found that DMN activity is measurably higher during rumination compared to a distraction control condition — meaning your brain isn't just "idling" while you replay something, it's showing elevated activity in this exact network. Other studies have found that the strength of functional connectivity within the DMN is directly associated with a person's tendency toward rumination — the more strongly connected these regions are at rest, the more prone someone is to getting stuck in looping thought patterns.

In other words: overthinking isn't a failure of willpower. It's a measurably more active, more tightly connected network doing more of exactly what it's built to do.

Autism adds another layer

Autism research has independently found default mode network differences too, with particular relevance to regions involved in social cognition and self-referential thought. A large-scale study analyzing resting-state brain scans from over 1,300 participants identified shared patterns of default mode network dysfunction across both ADHD and autism, and used those brain-based patterns to classify diagnoses more accurately than traditional categorical criteria alone — strong evidence that this isn't a vague, hand-wavy connection, but a measurable, shared neurological thread running through both conditions.

For a lot of AuDHDers, this combination may help explain something specific: it's not just generic overthinking, it's disproportionately social overthinking — replaying what you said in a meeting, dissecting a friend's tone, rehearsing a conversation that already happened. If autism-linked DMN differences are especially tied to self-referential and social processing, and ADHD-linked DMN differences make the whole network harder t

o quiet down, you end up with a network that's both more active and pointed disproportionately at social self-analysis.

Why this matters

None of this makes the looping thoughts feel less exhausting in the moment. But there's real value in understanding what's actually happening. This isn't you being "too sensitive" or unable to let things go through some personal failing. It's a specific, well-documented brain network — one that researchers can watch light up on a scan — behaving in a way that's consistent with what the science shows about ADHD and autistic brains generally.

That reframe matters because it changes the question. Instead of "why can't I just stop thinking about this," the more useful question becomes: what actually helps quiet an overactive default mode network? That's a mechanical question with mechanical answers — rather than a referendum on your character.

References

Harikumar, A., Evans, D. W., Dougherty, C. C., Carpenter, K. L. H., & Michael, A. M. (2021). A review of the default mode network in autism spectrum disorders and attention deficit hyperactivity disorder. Brain Connectivity, 11(4), 253–263. https://doi.org/10.1089/brain.2020.0865

[Author information not fully available]. Shared endo-phenotypes of default mode dysfunction in attention deficit/hyperactivity disorder and autism spectrum disorder. Translational Psychiatry. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6050263/

The subsystem mechanism of default mode network underlying rumination: A reproducible neuroimaging study. bioRxiv. https://www.biorxiv.org/content/10.1101/841239.full.pdf

Berman, M. G., Peltier, S., Nee, D. E., Kross, E., Deldin, P. J., & Jonides, J. (2011). Depression, rumination and the default network. Social Cognitive and Affective Neuroscience, 6(5), 548–555.