Brain damage can lead to mental retardation, as others have said.
Insecure attachment, from inadequate emotional support on the part of the parent, especially in the first three years, can lead to life-long insecurity and clingy or avoidant behaviour, as well as disorganized thinking. Someone posted a link to a document discussing the next DSM and it mentioned an east Asian country (Korea?) that is seeing an increase in this because of primary caregivers entering the workforce when their kids are really young, and how avoidant attachmentn patterns can be confused with autism (on the surface, anyway). Attachment theory started with John Bowlby and Mary Ainsworth and is summarized in Robert Karen's Becoming Attached.