Steven Pinker discusses the CPH in his book, The Language Instinct. According to Pinker, language must be viewed as a concept rather than a specific language because the sounds, grammar, meaning, vocabulary, and social norms play an important role in the acquisition of language. Physiological changes in the brain are also conceivable causes for the terminus of the critical period for language acquisition. As language acquisition is crucial during this phase, similarly infant-parent attachment is crucial for social development of the infant. An infant learns to trust and feel safe with the parent, but there are cases in which the infant might be staying at an orphanage where it does not receive the same attachment with their caregiver. Research shows that infants who were unable to develop this attachment had major difficulty in keeping close relationships, and had maladaptive behaviors with adopted parents.