Majority of people considers Vertical lines looks longer than horizontal ones when they see the above image in spite both the lines are of same length.
The vertical-horizontal illusion is the tendency for observers to overestimate the length of a vertical line relative to a horizontal line that has the same length. This involves a bisecting component that causes the bisecting line to appear longer than the line that is bisected. People often overestimate or underestimate the length of the bisecting line relative to the bisected line of the same length. This even happens if people are aware that the lines are of the same length.
There are several different configurations of the vertical–horizontal illusion. The three configurations which seem to produce the highest illusion magnitude are the L configuration, the plus (+) configuration, and the inverted-T configuration. Of these three, the inverted-T configuration produces the highest illusion magnitude. When the bisecting line of the T illusion is configured horizontally, the illusion magnitude is lowered. However, when the bisecting line of the T illusion is configured vertically, the illusion magnitude is higher.
A gradual decrease in error of vertical–horizontal illusions occur as participants age increases from eight to fourteen years. This decrease in error is attributed to the child’s improved ability to detect and de-center their attention in a visual display, i.e. position their body differently to gain other perspectives. Children who showed greater personal independence, verbal articulation, and visual scanning ability were more effective and resourceful in their ability to gauge vertical–horizontal illusions.
Source: Vertical-horizantal illusion