Elements moving in sync are perceived as one
Aliases: Gestalt common fate · common fate grouping
What it is
Elements moving in the same direction at the same speed get read as belonging to one object or one group, even if they're scattered across a static frame, differ in color and shape, and never touch. This cue is common fate, and it goes by whether the motion pattern matches, not by any static attribute.
What makes it distinct is that it's a temporal cue — every other Gestalt cue (proximity, similarity, closure) can be read from a single static frame; common fate requires watching a motion trajectory over time to judge, and a single frame gives no information at all about which elements "share a fate."
Why it happens
Deciding which elements belong to the same moving whole relies on comparing each one's velocity vector — whether direction and speed match — rather than comparing position itself. This comparison is carried out by a visual pathway dedicated to motion information, one that's relatively independent from the pathways handling color and shape, so even a set of dots with no relationship in color, size, or starting position will still be identified as one moving group and segregated from the background as long as their trajectories stay correlated — for instance, all rotating around a center at the same angular speed.
Studying it
The classic paradigm is the random-dot kinematogram: a subset of dots is given correlated motion (moving in a consistent direction, or all rotating around a shared center) while the rest move in random directions, and observers reliably identify the correlated subset as a coherent whole or figure — even though those dots are completely indistinguishable from the random ones in any single frame. Common independent variables are the proportion of dots carrying correlated motion and how consistent that motion is (the spread in direction or speed); the dependent variable is typically the proportion threshold needed for observers to reliably identify the figure, or their judgment accuracy.
The value of this paradigm is that it strips away every static cue except motion — there's nothing else in the display that could drive grouping — so once observers can reliably group the correlated dots, that establishes this cue works on its own, without help from color, shape, or position.
Where it stops holding
Common fate depends on the motion signal being reliably extractable — if the correlated motion is too small in magnitude, too slow, or swamped by stronger random-motion noise, this cue fails before other static cues do, because motion detection itself has its own sensitivity floor.
When several elements move at once but their direction and speed only approximately match rather than matching exactly, whether they're still grouped together is a matter of degree: the more precise the match, the more reliably they're grouped; once the mismatch becomes noticeable, the elements get read as independent objects on different trajectories — the specific consequences of that are covered in a sibling entry within this group.