Where does fear come from? As the jack-o’-lanterns show their grinning, glowing faces and skeletons, cobwebs, and gravestones adorn yards around the neighborhood, it’s a question hanging in many of our minds. When you recoil at the giant mechanical spider suspended above your neighbor’s garage, is that fear instinctual, or is it learned?

According to the Association for Psychological Science, there are only two fears we inherit at birth: the fear of falling and the fear of loud sounds.

A 1960 study, conducted by psychologists Gibson and Walk for Cornell University, sought to investigate depth perception in human and animal species. They suspended a sheet of transparent plexiglass about 4 feet off the ground and covered one half of it with a checkerboard-pattern cloth, creating a simulated cliff. Infants, both human and animal, were then encouraged by their caregivers, usually their mothers, to crawl off the “cliff” onto the clear half of the platform. Both avoided stepping over what they perceived as a sharp drop, and pre-crawling-age infants showed heightened cardiac distress on the “suspended” side.

Coupled with this innate fear of plummeting to the ground is something called the Moro reflex, one of several involuntary reflexes healthy newborn infants have at birth. Often called the “startle reflex,” it occurs when a baby is startled by a loud sound or movement, especially a falling motion. The reflex usually triggers the newborn to lift and spread their arms as if grasping for support, followed by crying. Though the Moro reflex usually disappears at around 5 to 6 months of age, our instinctive aversion to sudden loud noises stays with us throughout our lives.