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Method card · Visual illusion

Which line is longer?.

In four optical illusions, adjust two parts with a slider and stop exactly where they look equal.

Go to experiment2 min long8 real first answers

Original study

  1. Müller-Lyer, F. C. (1889). Optische Urtheilstäuschungen. Archiv für Physiologie, Supplement-Band, 263–270. Source
  2. Segall, M. H., Campbell, D. T., & Herskovits, M. J. (1963). Cultural differences in the perception of geometric illusions. Science, 139(3556), 769–771. (Full report: The influence of culture on visual perception. Indianapolis: Bobbs-Merrill, 1966.) Source
  3. Henrich, J., Heine, S. J., & Norenzayan, A. (2010). The weirdest people in the world? Behavioral and Brain Sciences, 33(2–3), 61–83. Source
  4. Gregory, R. L. (1963). Distortion of visual space as inappropriate constancy scaling. Nature, 199(4894), 678–680. Source
  5. Howe, C. Q., & Purves, D. (2005). The Müller-Lyer illusion explained by the statistics of image–source relationships. Proceedings of the National Academy of Sciences, 102(4), 1234–1239. Source
  6. Amir, D., & Firestone, C. (2025). Is visual perception WEIRD? The Müller-Lyer illusion and the cultural byproduct hypothesis. Psychological Review. Advance online publication. Source
  7. Bertamini, M., & Wade, N. J. (2023). Mario Ponzo (1928) on perception of numerosity: A translation and commentary. Perception, 52(8), 545–575. Source
  8. Leibowitz, H., Brislin, R., Perlmutter, L., & Hennessy, R. (1969). Ponzo perspective illusion as a manifestation of space perception. Science, 166(3909), 1174–1176. Source
  9. Yildiz, G. Y., Sperandio, I., Kettle, C., & Chouinard, P. A. (2022). A review on various explanations of Ponzo-like illusions. Psychonomic Bulletin & Review, 29(2), 293–320. Source
  10. Roberts, B., Harris, M. G., & Yates, T. A. (2005). The roles of inducer size and distance in the Ebbinghaus illusion (Titchener circles). Perception, 34(7), 847–856. Source
  11. de Fockert, J., Davidoff, J., Fagot, J., Parron, C., & Goldstein, J. (2007). More accurate size contrast judgments in the Ebbinghaus illusion by a remote culture. Journal of Experimental Psychology: Human Perception and Performance, 33(3), 738–742. Source
  12. Aglioti, S., DeSouza, J. F. X., & Goodale, M. A. (1995). Size-contrast illusions deceive the eye but not the hand. Current Biology, 5(6), 679–685. Source
  13. Franz, V. H., Gegenfurtner, K. R., Bülthoff, H. H., & Fahle, M. (2000). Grasping visual illusions: No evidence for a dissociation between perception and action. Psychological Science, 11(1), 20–25. Source
  14. Künnapas, T. M. (1957). The vertical-horizontal illusion and the visual field. Journal of Experimental Psychology, 53(6), 405–407. Source
  15. Prinzmetal, W., & Gettleman, L. (1993). Vertical-horizontal illusion: One eye is better than two. Perception & Psychophysics, 53(1), 81–88. Source
  16. Chouinard, P. A., Peel, H. J., & Landry, O. (2017). Eye-tracking reveals that the strength of the vertical-horizontal illusion increases as the retinal image becomes more stable with fixation. Frontiers in Human Neuroscience, 11, 143. Source

Our adaptation

Measures Müller-Lyer (1889) and three other classic geometric illusions by the method of adjustment. In round 1 a fixed shaft with outward fins sits on top and a shaft with inward arrowheads below; the lower shaft is adjusted with a slider until both look equal. Round 2 (Ponzo), round 3 (Ebbinghaus) and round 4 (vertical-horizontal) are adjusted the same way. The slider starts from a random value on the short or long side and no numbers are shown.

Differences from the original study

  • Each illusion is adjusted once; laboratory studies average several trials started from both directions.
  • Screen size and viewing distance are not controlled.
  • The four illusions always come in the same order.

Measured variables

The columns of the open data file. List fields are expanded into numbered columns (for example rt_60, est_3); JSON columns contain only numbers and fixed stimulus names.

ColumnTypeDescription
rationumberRound 1 (Müller-Lyer): adjusted lower shaft / fixed upper shaft (1 = true equality; shaft lengths only, fins excluded).
illusion_pctnumberRound 1 illusion magnitude: (ratio − 1) × 100; a positive value means the lower line was left longer.
ponzonumberRound 2 (Ponzo): adjusted / true ratio (1 = equal).
ebbnumberRound 3 (Ebbinghaus): adjusted / true ratio (1 = equal).
vhnumberRound 4 (vertical-horizontal): adjusted / true ratio (1 = equal).
Columns present in every file (14)
row_idstringRow code. Regenerated at random in every release; it does not identify a person and cannot be matched across releases or experiments.
experimentstringShort name of the experiment (URL slug).
experiment_versionintegerVersion of the answer format. For experiments whose format changed, only the current format is published.
dateYYYY-MM-DD | YYYY-Www | YYYY-MMDay the answer was given (Istanbul time). If fewer than 5 answers share the same day, language and device, it is coarsened to the ISO week, and to the month if that is still too few.
date_precisionday | week | monthPrecision of the date column.
langtr | en | (boş)Interface language. Language recording started in the first week of October 2026; for earlier answers it is known only in experiments whose stimuli depend on the language, and empty otherwise.
devicedesktop | mobile | tablet | (boş)Device class reported by the browser (class only; browser details are neither stored here nor published).
sourcesite | embedWhere the answer came from: the balabs site or the experiment embedded on another site. The embedding site is not published.
modefree | daily | challenge | race | session | (boş)Play mode in scored experiments: free play, daily round, challenge, race room, experiment session. Empty for unscored experiments and for older answers that did not store it.
color_visionnormal | rg | by | contrast | (boş)In color-based experiments, the color vision mode the participant chose: normal, red-green, blue-yellow or high contrast. Stimuli are generated along different axes per mode, so compare within a mode. Empty for other experiments.
first_play1Every row is a participant’s first play of this experiment (always 1). Replays are stored only as leaderboard scores and are not part of the open data.
seen_beforeyes | no | unsure | (boş)Participant’s own report of whether they had seen this experiment or its known answer before (in experiments that ask).
duration_snumberTime from the start of the experiment to submitting the answer, in seconds (measured in the browser, 0.1 s).
play_scorenumberLeaderboard score of the first play (scored experiments; defined on the method card). Empty for unscored experiments.

Exclusion criteria

The server accepts a round-1 ratio between 0.4 and 2.0. The extra rounds are measured in the browser. We recommend setting aside adjustments with an absolute error above 50%.

  • Only each participant's first answer to this experiment; replays are not included.
  • Simulated rows, bots, banned and sample accounts are not included.
  • Answers before 3 October 2026 are not included (the first day the open data notice was live).

Scoring

Leaderboard score: the sum of the absolute illusion percentages over four rounds (Σ |ratio − 1| × 100); lower is better.

Leaderboard measure: total illusion (lower is better). The play_score column in the open data is this score for the first play.

Version notes

  • October 2026

    This experiment started storing the play mode (free, daily, challenge, race, session) with the answer; the mode column is empty for earlier answers.

  • October 2026

    The interface language (lang) started being stored with each answer; for earlier answers it is known only in experiments whose stimuli depend on the language, and empty otherwise.