Can seeing a small number nudge your hand to the left?
When people were free to flick a ball left or right after seeing a number, the small digit 2 made them choose the left goal more often than chance and also changed the timing of their movement.
Source
Act on Numbers: Numerical Magnitude Influences Selection and Kinematics of Finger Movement
Study at a glance
- Design
- Human experiment — Within-subject free-choice task: participants flicked a ball with a finger toward a left or right goal after arrows (forced direction) or numbers (free choice), with 3D motion tracking of the finger
- N
- N=19 · 19 right-handed university students, each completing 100 test trials
- Population
- Right-handed young adult university students in Padova, Italy
- Outcome
- Proportion of left kicks after small vs large numbers; movement kinematics (movement time, timing of maximum trajectory deviation, preparation and finalization phases) compared with arrow baselines
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What they did
Nineteen right-handed students wore a tiny soccer shoe on their right index finger and flicked a small ball toward one of two goals, one on the left and one on the right. On most trials an arrow told them which way to kick; on the rest they saw a small or large number (the digit 2 or 8, or an array of 2 or 8 dots) and could kick either way. Motion cameras tracked the finger so the researchers could split each kick into a preparation phase and a finalization phase and compare them with arrow-cued kicks.
What they found
Small numbers led to more left kicks than large numbers, and digits produced more left kicks than dot arrays. Only the digit 2 pushed left-kick choices reliably above chance; the digit 8 and both dot arrays did not. Kinematically, left kicks after the digit 2 and right kicks after the digit 8 both showed a longer preparation phase and a shorter finalization phase than baseline, whereas the dot arrays did not change the movement at all.
The limits
What it doesn't show
With only 19 participants and just 10 trials per number type, the study has limited power, and the authors acknowledge the sample could be seen as too small. Large numbers did not produce a clear rightward choice bias, partly because the right index finger's anatomy made left kicks easier overall, so the classic left-small / right-large pattern was only half confirmed in choices. The finding applied to digits but not to dot arrays, so it does not show that non-symbolic quantity biases action in the same way. The task is novel, so it is untested how well these effects generalize to other actions.
Key terms
- Mental number line (MNL)
- The idea that people represent numbers spatially, typically with small numbers on the left and large numbers on the right in Western cultures.
- SNARC effect
- Spatial-numerical association of response codes: people respond faster to small numbers with the left side and to large numbers with the right side.
- Free-response task
- A task where participants choose which of two responses to make, so any bias in which response they pick can be measured.
- Kinematics
- Detailed measurement of how a movement unfolds over time and space, such as its duration and when the trajectory deviates most.
- Symbolic vs non-symbolic number
- A number shown as a digit (symbolic) versus a quantity shown as a set of items like dots (non-symbolic).
Flashcards
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Quiz yourself
Which stimulus reliably increased left kicks above chance?
Common questions
Why use such a strange action as kicking a ball with a finger?
Everyday actions such as pressing keys or using rulers may carry learned links between number and space. A new, unfamiliar action reduces the chance that any bias simply reflects overlearned habits.
Why were arrows mixed in with the numbers?
Arrows gave a baseline for how a left or right kick normally unfolds, and making free choices rare kept participants from adopting a fixed strategy for number trials.
Did large numbers push people to kick right?
Not reliably in their choices: after large numbers, left kicks were at chance. But the timing of right kicks after the digit 8 did change, showing an effect on how the action was carried out.
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