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But what happens when babies spend a night or more per week away from a primary caregiver, as increasingly happens in cases where the parents share custody, but do not live together?
In a new national study, University of Virginia researchers found that infants who spent at least one night per week away from their mothers had more insecure attachments to the mother compared to babies who had fewer overnights or saw their fathers only during the day.
The finding is reported in the August edition of the Journal of Marriage and Family.
Attachments are defined as an enduring, deep, emotional connection between an infant and caregiver that develops within the child's first year of life, according to Samantha Tornello, the study's lead author and a Ph.D. candidate in psychology in U.Va.'s Graduate School of Arts & Sciences.
She notes that growing numbers of parents are living apart due to non-marital childbirth, the breakup of co-habitating parents, separation and divorce.
Parents increasingly are choosing to share child rearing in some form of joint custody, and often the legal system must determine custody arrangements for the children of parents who do not live together.
"Judges often find themselves making decisions regarding custody without knowing what actually may be in the best interest of the child, based on psychology research," Tornello said.
"Our study raises the question, 'Would babies be better off spending their overnights with a single caregiver, or at least less frequently in another home?'"
Tornello pointed out that either the mother or father could be the primary caregiver, but the point would be that the child ideally would be in the care each night of a loving and attentive caregiver and that there may be something disruptive about an infant spending nights in different homes.
"We would want a child to be attached to both parents, but in the case of separation a child should have at least one good secure attachment," she said. "It's about having constant caregivers that's important."
Tornello and her co-authors at U.Va. and the American Institutes for Research, including U.Va. psychology professor Robert Emery, analyzed data from the Fragile Families and Child Wellbeing Study, a national longitudinal study of about 5,000 children born in large U.S. cities from 1998 to 2000.
Additional in-home assessments of the children were conducted when they were 1 and 3.
Of parents who were not cohabiting at the time of the study, 6.9 percent of babies under the age of 1 and who lived primarily with their mother spent at least one overnight a week away with their father.
Among toddlers ages 1 to 3, 5.3 percent spent between 1 percent and 35 percent of overnights away with their fathers. Another 6.8 percent spent 35 percent to 70 percent of overnights with their fathers.
Infants who spent at least one overnight a week away from their mothers were discovered to have more insecure attachments to them compared to babies who had fewer overnights or stayed with their father only during the day.
Forty-three percent of babies with weekly overnights were insecurely attached to their mothers, compared to 16 percent with less frequent overnights.
In the case of toddlers the findings were less dramatic; greater attachment insecurity was linked to more frequent overnights, but the findings there were not statistically reliable, Tornello said.
"I would like infants and toddlers to be securely attached to two parents, but I am more worried about them being securely attached to zero parents," said Emery, Tornello's research adviser.
He advocates parenting plans that evolve, where day contact with fathers occurs frequently and regularly, and overnights away from the primary caregiver are minimized in the early years, then are gradually increased to perhaps become equal in the preschool years.
"If mothers and fathers can be patient, cooperate and take a long view of child development, such evolving plans can work for both children and parents," he said.
Journal Reference: Samantha L. Tornello, Robert Emery, Jenna Rowen, Daniel Potter, Bailey Ocker and Yishan Xu. Overnight Custody Arrangements, Attachment, and Adjustment Among Very Young Children. Journal of Marriage and Family, 1 JUL 2013; Volume 75, Issue 4, August 2013, Pages: 871%u2013885 DOI: 10.1111/jomf.12045