Illusions and Child Development: When Do Kids See Them?

Do children see optical illusions? It depends on the illusion and the child's age. Perception isn't finished at birth. Many of the abilities illusions depend on develop gradually, some within the first year and others still maturing through the school years, so illusions give an unusually direct view of how perception grows up.

Babies see more than you'd think

Very young babies have more working visual machinery than older theories assumed. Within weeks of birth they show basic sensitivity to contrast and edges, the early processing described in the role of the visual cortex. Because babies can't describe what they see, researchers infer it from where and how long they look: babies tend to look longer at something new than at something familiar, and to look at patterns they can tell apart from a blank field.

Studies using these methods show that some grouping abilities, the kind described in Gestalt principles of perception, appear early. Kavšek's review of the research found that babies within the first year respond to illusory contours like those of the Kanizsa triangle, treating them as real edges (Kavšek, 2002). Some of the brain's organising tendencies, it seems, don't need years of experience to get started.

Context takes longer

Other abilities take much longer, and illusions that depend on them can be weaker in young children than in adults. The Ebbinghaus illusion is a clear case. Doherty and colleagues found that while adults were strongly fooled by it, young children were barely affected and judged the sizes more accurately (Doherty et al., 2010). Káldy and Kovács found something similar in a related task: four-year-olds were less influenced by the surrounding circles than adults, suggesting that the ability to integrate visual context isn't fully developed at that age (Káldy & Kovács, 2003).

That runs against the popular idea that children, as more "innocent" observers, must be more easily fooled. For illusions that depend on taking in the whole scene, it's often the other way round: children are fooled less because they haven't yet built the habits adults use automatically.

Not every illusion follows the same path

Illusions don't all change with age in the same direction. The Ebbinghaus and Ponzo illusions generally grow stronger through childhood, while the Müller-Lyer illusion has generally been found to weaken with age. The results also depend on how the illusion is measured: asking children to adjust a line until it looks right can give a different answer from asking which of two lines is longer. That variety is itself informative. If different illusions follow different timetables, they probably depend on different processes that mature separately.

A layered system

The development of illusions supports a point that runs through vision science: perception isn't one ability that switches on, but a stack of separate mechanisms, some largely built in and quick to mature, others depending on years of experience with a world full of perspective, shadows and context. Watching how illusions change through childhood is a way of watching that stack come together. The same theme appears in how illusion strength varies between adults, covered in do optical illusions work the same for everyone, and between species, covered in do animals see optical illusions.

Sources and further reading

1. Kavšek, M. J. (2002). The perception of static subjective contours in infancy. Child Development, 73(2), 331–344. Find this reference on Google Scholar
2. Káldy, Z., & Kovács, I. (2003). Visual context integration is not fully developed in 4-year-old children. Perception, 32(6), 657–666. Find this reference on Google Scholar
3. Doherty, M. J., Campbell, N. M., Tsuji, H., & Phillips, W. A. (2010). The Ebbinghaus illusion deceives adults but not young children. Developmental Science, 13(5), 714–721. Find this reference on Google Scholar