Inattentional blindness
An unexpected object in plain view goes unseen when attention is committed elsewhere.
When someone is engaged in an attention-demanding task, an unexpected stimulus can appear in full view and not be consciously perceived. It differs from change blindness: there is no disruption and nothing changes, the object is simply outside the focus of attention. The heavier the primary task, the more likely the miss.
How it shows up in software
A person filling in a form is running a demanding task, which is exactly when a warning, a price change or a consent notice placed elsewhere on the page will be missed. Users are not being careless. Anything that must be seen has to be placed on the path of the task, not beside it.
Using it well
- Put critical information in the flow of the task: above the button that acts on it, in the same column, at the moment of decision.
- Interrupt for things that genuinely cannot be missed, and reserve interruption so it retains force.
- Test with people doing the real task under time pressure, since a participant told to look at the page sees everything.
- Repeat load-bearing facts at the point of commitment rather than trusting an earlier statement to have landed.
Where it turns manipulative
- Placing a disclosure, an auto-renewal term or a fee where you know an engaged user will not look, then treating it as disclosed.
- Using the effect to smuggle a pre-ticked option past someone mid-task.
- Inflating interruption for routine messages, which pushes people to dismiss reflexively and removes the channel you need for real warnings.
Where you have seen it
Stripe Checkout
The amount being charged sits directly above the pay button rather than in a header the person has scrolled past.
Gmail
Mentioning an attachment without attaching one triggers a blocking prompt at send, at the moment of commitment.
iOS
Permission requests are modal and arrive when the feature is first used, rather than sitting in a settings page.
What the research says
- Mack and Rock, 1998Well evidenced
Across many experiments, participants performing a demanding judgement failed to report an unexpected shape presented at the same time, including near fixation.
- Simons and Chabris, 1999Well evidenced
Participants counting basketball passes frequently failed to report a person in a gorilla suit walking through the scene. Misses were more common when the counting task was harder.
- Drew, Vo and Wolfe, 2013Mixed evidence
Most expert radiologists searching scans for nodules failed to report a gorilla image inserted into the scan, showing expertise does not protect against the effect.
Twenty-four radiologists in one study. It illustrates the effect in a professional task rather than quantifying clinical risk.
Grades are a judgement about the evidence, not about how useful the idea is. Plenty of contested effects are still worth knowing, as long as you do not cite them as settled.