According to Gregory (1966), susceptibility to the Müller-Lyer illusion results from:

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Gregory proposed that living in a carpentered environment, where we are constantly exposed to architectural features with right angles and corners, leads to a cultural conditioning of the visual system that causes susceptibility to the Müller-Lyer illusion. This suggests that susceptibility is not hard-wired or genetically determined, but rather a result of environmental factors. Visual affordances and motivational factors are not directly related to susceptibility to the Müller-Lyer illusion.

According to Gregory (1966), Susceptibility to the Müller-Lyer Illusion Results