Piaget came to his conclusion that infants lacked a complete understanding of object permanence before 18 months after observing infants' failure before this age to look for an object where it was last seen. Instead, infants continue to look for an object where it was first seen, committing the "A-not-B error." Some researchers have suggested that before the age of eight to nine months, infants' inability to understand object permanence extends to people, which explains why infants at this age do not cry when their mothers are gone ("Out of sight, out of mind").