"Potty training" for defecation is an extremely tense subject for many families. Everyone wants his or her little one to use the toilet as soon as possible. Requirements for child care facilities and the unsavory task of handling stool-filled diapers makes this something to achieve at the earliest age possible.
In the past, pediatric behaviorists have tried to link toilet refusal to significant behavioral problems. More recent studies show that children who refuse to defecate in a toilet by age four years do not have more behavior problems than those who are trained earlier. They also noted a higher incident of constipation and pain with bowel movement passage.
In my own patient experience, it seems that toilet use refusal for bowel movements is more common in family situations where parents exert great emotional pressure to "perform on the pot." The pressure can either be positive or negative. Today few people feel it is sound to punish a child for not using a toilet. In the same breath I would like to say, reward your child for using the toilet only if someone rewards you for a similar performance. Let toilet usage be as neutral and natural as possible.