My answer may sound like heresy coming from a man who spent ten years of his life as a professor of pediatrics, responsible for medical and behavioral research, but I don't believe the scientific community is capable of determining the best parenting techniques. There have been some worthwhile studies, to be sure, but the subject of discipline almost defies definitive investigation.
Why? Because the only way to study this topic scientifically would be to place newborns randomly in "permissive" vs. "disciplined" families and then keep them under close observation for ten or fifteen years. Since it is impossible to do that, researchers have tried to tease out information where they could find it. But family relationships are so multidimensional and complicated that they almost defy rigorous scrutiny. Indeed, most of the studies reported in the literature are scientifically useless. For example, Dr. David Larson, psychiatrist and formerly a researcher at the National Institutes of Health, reviewed 132 articles in professional journals that purported to investigate the long-term consequences of corporal punishment. He found most of them flawed in design. Ninety percent of the studies failed to distinguish between good homes where spanking was administered by loving parents, and those bordering on (or actually inflicting) child abuse. This distinction is critical for obvious reasons. Dr. Larson concluded that the findings were invalidated by this failure to consider the overall health of family relationships.1
To repeat, the consequences of various approaches to parental discipline appear to be beyond the reach of social research. It is simply not possible to study this complex subject scientifically without warping families to set up the research design. Even if such studies were conducted, the researchers would be studying contrived families--not typical parent-child relationships.
Dr. David Larson, “Is Mild Spanking Abusive or Helpful for Young Children?” Physicians Research Forum Research Summary, 1993.