MotivationAlso called Intermittent reinforcement, Variable ratio schedule

Variable reward

Unpredictable payoffs hold behaviour longer than reliable ones, and resist stopping.

When a reward arrives on an unpredictable schedule, the behaviour that produces it is repeated at a high rate and persists after rewards stop. Predictable rewards produce steadier but more fragile behaviour, because their absence is immediately obvious. Variability removes the signal that tells someone to quit.

Variable schedules resist stopping

Schematic of the classic operant finding. Once rewards stop, responding under a fixed schedule falls away quickly, because the missing reward is obvious. Under a variable schedule it persists, because absence looks like a normal gap.

How it shows up in software

Pull to refresh, algorithmic feeds, notification batches, match queues and loot systems all deliver payoff on a schedule the user cannot predict. The user cannot tell whether the next check is worth it, so checking becomes the cheapest way to find out. The mechanic is what turns a feed into something people open without deciding to.

Using it well

Let variability come from genuine supply, like new messages actually arriving, instead of withholding content to time its release.
Give a clear empty state: What a screen shows before there is any content in it. so a user who checks and finds nothing gets a real answer and can leave.
Batch and schedule notifications on a rhythm the user sets, which removes the unpredictability you did not need.
Track session frequency alongside session satisfaction, and treat rising frequency with flat satisfaction as a warning rather than a win.

Where it turns manipulative

This is the core machinery of compulsive products. Deliberate uncertainty applied to attention produces checking that users report they do not want and cannot easily stop, and the harm concentrates in teenagers and in people already struggling with impulse control.
Paid variable reward: An unpredictable payoff, which sustains a behaviour longer than a predictable one. is gambling. Loot boxes and pull mechanics sold for money take the schedule that resists extinction and attach a payment to each pull, which is why several jurisdictions regulate them.
Artificial scarcity, holding back content that already exists so it can drop unpredictably, spends the user's time to buy your session count.

Where you have seen it

Instagram
Likes and notifications surface in batches rather than at the moment each one occurs.
Tinder
Matches arrive on an unpredictable schedule as swiping continues, with no way to tell if the next swipe pays.
Genshin Impact
Character pulls resolve against a published probability table with a pity counter that guarantees a rare result eventually.

What the research says

Ferster and Skinner, 1957Well evidenced
Variable ratio schedules produced the highest and steadiest response rates and the slowest extinction of the schedules tested.
Pigeons and rats in operant chambers. The pattern is one of the most reliable findings in behaviour analysis, and its transfer to humans choosing what to open on a phone is an inference, not a measurement.
Fiorillo, Tobler and Schultz, 2003Mixed evidence
Dopamine neurons in monkeys showed sustained activity during the delay before a reward of uncertain probability, peaking at fifty percent odds.
This is the study most popular writing means by a dopamine hit, and it does not say what that writing claims. It recorded anticipation signalling in monkeys, not pleasure, not addiction, and nothing about phones. Treat any product argument resting on dopamine as pop science until it cites a human behavioural result.

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.