  Not only do these representations support comparisons of magnitude, they have also been shown to support approximate addition and subtraction in babies as young as 9-months-old. In one study, babies were presented with a set of 5 objects that moved behind a screen so they were no longer visible. Then another set of 5 objects was presented and they too moved behind the screen. When the screen was removed, babies looked longer if there were only 5 objects than if there were 10. In a parallel subtraction condition, where babies first saw 10 objects move behind the screen, and then saw 5 objects taken away, they stared longer when the screen was removed to show a display of 10 objects (McCrink &amp; Wynn, 2004a).