Almost half of all American children must cope, at some point in their lives, with the disintegration of their parents' marriage. A controversial new study by psychiatrist Judith Wallerstein contends that the children of divorce are much more likely to be troubled as adults and that couples with kids need to try harder to remain married. But critics describe the study as unscientific and argue that bad marriages often end up doing more harm than good to the whole family. Child-advocacy experts also disagree over the impact of custody arrangements. Some favor joint physical custody because they say it allows both parents to remain involved in their children's lives. But others say that forcing a child to live in two homes is disruptive and makes an already difficult situation worse.