There is a kind of yes that comes out of a hard afternoon faster and fuller than you meant it to. Someone asks for something. You hear yourself agreeing, to the meeting, the favour, the extra thing, before you have done the part where you actually weigh whether you can. You come home having committed to more than you would have chosen with a clear hour and a quiet room. Nothing was snapped. No one was short with you. You just said yes to the version of the day that needed a week's consideration, and the considering never quite happened.

We wrote last week about a related but different thing: the stopping mechanism under pressure, the moment before a reaction that seems to narrow when stress is in the body. That post was about the brake on something already in motion. This one is about a layer underneath it: not the reaction that escapes, but the quality of the deliberation that was supposed to happen first. What goes wrong before the moment of stopping ever arrives.

The choosing doesn't stop. It moves.

The clearest study I have found on this is a 2025 randomised trial, small but carefully built.[1] Researchers stressed people in the lab and then had them work through a genuinely hard decision task: a problem that requires holding several considerations at once and weighing them, the kind of thinking that cannot be done on reflex. They measured cortisol and tracked the eyes while people decided.

What they found is specific. Higher cortisol predicted worse decisions, and the damage was concentrated on the hardest problems. The easy ones held. It was the choices that genuinely needed weighing that came apart. And the most severe drop, roughly twenty-five percentage points of accuracy, happened when stress and time pressure arrived together. Under that combination, people were also more likely to time out: to use up all the available time without landing on a decision at all, the deliberation stalling rather than resolving.

That is a different finding from a slow brake. It is the difference between failing to stop a reaction and failing to do the careful weighing in the first place.

A shift in who is doing the deciding

The mechanism, as best the field can describe it, is not that stress switches off the deciding. It moves it.

An integrative review of the neurobiology lays out the shape of it.[2] Under acute stress, high levels of noradrenaline and dopamine disrupt the prefrontal cortex, the slow, effortful system that holds competing considerations in mind and resists the obvious answer. At the same time, the more reactive, pattern-driven parts of the brain get louder. Control shifts from goal-directed evaluation toward fast, habitual, stimulus-driven responding. The choosing continues. It is just handed to a part of the brain that runs on older patterns and does not pause to weigh.

This is a review, not an experiment, so it is the right place to take the why and not a specific number. But it fits the trial's finding cleanly. If the weighing system is the part that goes offline under load, then the choices that need weighing are exactly the ones that should fall apart, while the easy, automatic ones stay intact. Which is what the data showed.

One careful step further, and this is an inference rather than a measured result: over-committing may be what this shift looks like from the inside. When the system that holds do I actually have room for this goes quiet, the faster system reaches for the familiar move, and for many people the familiar move is yes. No study found here measured over-commitment directly in daily life. It is a plausible reading of the mechanism, offered as that and not as proof.

Who this has been shown in

I want to be careful here, in the same place the last post was careful. The decision-quality trial did not break its results down by sex, and the sample was too small to. The review treats sex as a factor that matters but does not give sex-specific findings. So I cannot tell you this has been demonstrated in women, and I am not going to imply it has.

There is one older, smaller study that points at a sex difference, and it is worth naming precisely because it is uncertain.[3] It found that higher stress-hormone levels predicted more impulsive choosing, a stronger pull toward the immediate option, in women, with the opposite direction in men. It is a 2011 preprint (n=87) that has not been widely replicated. Hold it loosely. It is a hint that the picture might look different for women, not a result you can lean any weight on.

Where this lands

You do not have to do anything with this. There is no rule here about when to make decisions, no window to protect on the calendar. The evidence on the specific over-commitment pattern is softer than what we covered last week, and that is worth saying plainly rather than dressing up.

But the next time you find you have agreed to more than you had room for, after a pressured stretch, with the clock running, it might be worth holding a different account than the one that reaches for I have no boundaries or I can never say no. Under stress and time pressure together, the part of the brain that does the weighing is the part that goes first. The yes that arrived before you had decided is not evidence of a weak character. It is what a deliberation system looks like when it has briefly been handed to a faster, older one.

Noticing that an over-full week connects back to the pressured, time-squeezed hours where the agreeing happened, so a pattern stops reading as a flaw in you and starts looking like a system under load. That is the kind of thread MoMo was built to hold across days. But that is a footnote. The essay is the weighing, and where it goes under pressure.

If any of this lands in a heavy or frightening way, or if you're in distress or crisis, please reach out: Mind Sweden 90101 | Samaritans UK 116 123 | your local crisis line.

References

Doroc, K., Yadav, N., & Murawski, C. (2025). Acute stress impairs decision-making at varying levels of decision complexity. Communications Psychology, 3, 135. PMID: 41315734.
Sarmiento, L. C., Lopes da Cunha, P., Tabares, P., Tafet, G., & Gouveia Jr, A. (2024). Decision-making under stress: A psychological and neurobiological integrative model. Brain, Behavior, & Immunity - Health, 38, 100766. PMID: 38694793.
Stress hormones predict hyperbolic time-discount rates six months later in adults (2011). arXiv:1111.6489.

Stress doesn't change the feeling. It changes the brake. (momocompanion.com/blog/stress-kills-the-pause-not-the-feeling) covers the stopping mechanism: a distinct layer from the deliberation this post is about, not a restatement of it.