The best way to lift the condition of a litter is to reward excellence in all puppies, while providing pathways of opportunity for the least advantaged ones. The least advantaged puppies will ultimately benefit most. This is the promise of litter enrichment – the notion that a rising tide raises all ships. Advanced breeders are acutely aware of the critical periods of development that a puppy goes through. They leverage those critical periods by providing the right environment and socialization to benefit each individual puppy.
Dr. John Paul Scott and John Fuller are credited with the most comprehensive study of dog behavior ever published. In my opinion, anyone who breeds dogs should become familiar with their research.
Scott, J.P., and John L. Fuller. 1974. Genetics and the Social Behaviour of the Dog. A Phoenix Book, P570. University of Chicago Press. http://esbt.us/2y.