http://www.minddisorders.com/Del-Fi/Enuresis.html
Causes in children
The fourth edition of the Diagnostic and Statistical Manual of Mental Disorders , fourth edition, text revision, or ( DSM-IV-TR ), does not distinguish between children who wet the bed involuntarily and those who voluntarily release urine. Increasingly, however, research findings suggest that voluntary and involuntary enuresis have different causes.
Involuntary enuresis is much more common than voluntary enuresis. Involuntary enuresis may be categorized as either primary or secondary. Primary enuresis occurs when young children lack bladder control from infancy. Most of these children have urine control problems only during sleep; they do not consciously, intentionally, or maliciously wet the bed. Research suggests that children who are nighttime-only bed wetters may have a nervous system that is slow to process the feeling of a full bladder. Consequently, these children do not wake up in time to relieve themselves. In other cases, the child's enuresis may be related to a sleep disorder.
Children with diurnal enuresis wet only during the day. There appear to be two types of daytime wetters. One group seems to have difficulty controlling the urge to urinate. The other group consciously delays urinating until they lose control. Some children have both diurnal and nocturnal enuresis.
Secondary enuresis occurs when a child has stayed dry day and night for at least six months, then returns to wetting. Secondary enuresis usually occurs at night. Many studies have been done to determine if there is a psychological component to enuresis. Researchers have found that secondary enuresis is more likely to occur after a child has experienced a stressful life event such as the birth of a sibling, divorce or death of a parent, or moving to a new house.
Several studies have investigated the association of primary enuresis and psychiatric or behavior problems. The results suggest that primary nocturnal enuresis is not caused by psychological disorders. Bed-wetting runs in families, however, and there is strong evidence of a genetic component to involuntary enuresis.
Unlike involuntary enuresis, voluntary enuresis is not common. It is associated with such psychiatric disorders as "oppositional defiant disorder" , and is substantially different from ordinary nighttime bed-wetting. Voluntary enuresis is always secondary.