Harlow (1959) initially believed that being mothered was the critical experience that would allow a monkey to become an adequate mother, but subsequently, a series of studies revealed that other social experiences could substitute for being mothered. Harlow and his associates discovered that physical contact was an important factor in learning "mothering" They constructed two types of surrogate mothers, one a wire "mother" and one a cloth-covered wire "mother" Neither type of surrogate was very much like a real monkey's mother, each did not move, hold the infants, or respond to them in any way. In some of Harlow's studies, both types of surrogates offered milk for nursing, whereas in some conditions, only the wire surrogate offered milk. Although the surrogate mothers were unresponsive, the infants were not. The infants strongly preferred the cloth-covered surrogates to the wire surrogates, even if the wire surrogate was the sole source of food. Infants nurse from the wire "mother" but clung to the cloth-covered surrogate for hours and ran to it when frightened. Harlow concluded that the cloth-covered surrogates provided some comfort that the wire surrogates could not. and he called this factor contact comfort, the security provided by physical contact with a soft, caring, or comforting object. However even these monkeys did not become socially, sexually or maternally normal, indicating that the cloth surrogate had failed to provide the experiences that are necessary for normal monkey development.