In an ideal world, at least an ideal scientific--even if ethically horrible--one, the task would be rather straightfoward: Simply decide, on a random basis, that some children will be parented, for instance, in a warm, sensitive, emotionally supportive and intellectually stimulating manner and that others will be cared for in just the opposite way. Then follow the children up at a later age and make comparisons between them. Any differences would be attributable to effects of early rearing.
For this research to yield truly compelling answers to questions about enduring influences of parenting early in life on development later in life, the skilled scientist would not even rely on biological parents to rear their own children--in the manner the scientist dictates--but have other, unrelated adults do the job. This kind of investigatory freedom is engaged in all the time when animals are the focus of investigation. Some such work, for example, takes rats or monkeys away from their biological mothers and hands them over to another adult (of the same species) who is known, on the basis of prior study of their parenting skills, to care for offspring very well or very poorly. An alternative strategy is to induce the adult who provides foster care to the young animal to care for their pseudo-progeny in a sensitive or insensitive manner by modifying the conditions under which parent and child live--by, for example, creating crowded or comfortable conditions, by providing sufficient food or doing the opposite, and so on and so forth. Once again, comparisons between animals cared for well or poorly early in life allow one to discern enduring effects of early care when the animals are studied later in life.