Here's a citation of the most recently published large study on the psychological effects of bed-sharing with children:
Quote:
Title: Outcome correlates of parent-child bedsharing: An eighteen-year longitudinal study
Author(s): Okami P, Weisner T, Olmstead R
Source: JOURNAL OF DEVELOPMENTAL AND BEHAVIORAL PEDIATRICS 23 (4): AUG 2002
Document Type: Article
Language: English
Abstract: We report results of the first longitudinal study of outcome correlates of parent-child bedsharing. Two hundred five families in nonconventional and conventional family lifestyles have been followed since 1975. A target child in each family was followed from the third trimester of mother's pregnancy through age 18 years. Bedsharing in early childhood was found to be significantly associated with increased cognitive competence measured at age 6 years, but the effect size was small. At age 6 years, bedsharing in infancy and early childhood was not associated with sleep problems, sexual pathology, or any other problematic consequences. At age 18 years, bedsharing in infancy and childhood was unrelated to pathology or problematic consequences, nor was it related to beneficial consequences. We discuss these results in light of widespread fears of harm caused by parent-child bedsharing. We suggest that such fears are without warrant if bedsharing is practiced safely as part of a complex of valued and related family practices.
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This is a large, careful, long term study. It concludes that fears about the psychological effects of bed sharing during early childhood on children are unfounded, while demonstrating that there are small benefits on cognitive function measured at age 6.
So I don't think there's any problem for small children (to age 3-5). The only concern there really is the privacy of the parents.
Cosleeping with older children though is so rare that there don't seem to be any studies of its effects.