William Sears postulates the existence of a brief time slot immediately after birth during which the newborn is in a "quiet alert state" and particularly accessible for bonding. He refers to this birth bonding as "imprinting" and bases himself on a study by Marshall Klaus and John Kennell from 1967; however, Klaus and Kennell later modified their original assumptions, including the one cited by Sears.[25] Sears advises women to abstain from analgesics during childbirth, since those drug the child, too, and according to Sears interfere with the birth bonding.[26]