Developmental psychologists have attempted to identify the effects of many specific childrearing practices on children's personality traits. the problem with this line of research, which was discussed in the text, is that

that parents are inconsistent from day to day and over the years.

Explanation:

Based on the information provided within the question it can be said that the problem with this line of research is that parents are inconsistent from day to day and over the years. This leads to various results and eliminates the possibility of validity within the research. In order for the results to be valid, the inputs as well as the outcomes of the research must be consistent.