The quotation marks are my own addition. This principle taps into a few main concepts in cognitive psychology. Each of these concepts have many papers attributed to each of them individually, but I've decided to give a very short summary of each in order to justify the authors' conclusions.

• Cognitive Load Theory: there is an upper bound to how many pieces of information one can keep in mind to the end of problem solving.
• Information Overload: when people are presented with too much relevant information, they struggle to use any analytical tools and will often instead turn to analytical "coping mechanisms" such as tuning out.
• Working Memory Theory of Learning: in order for information to be learned (and retained) information needs to first be processed by one's working memory before it can be processed by their long-term memory. Therefore, if there is too much information for working memory to hold all at once, there is no chance that it will be saved in long term memory.

Students learn more when educators teach "less" and focus on teaching fewer things. Cook and Klipfel advise educators to write down a list of possible outcomes for students after receiving a given lesson, (ie Students learn A. Students learn B. Students are prepared to learn about C) and choose the top 2 or 3 that they will focus on. Other outcomes can be used to direct discussion, but the educator would ideally focus the entire lesson on just a few outcomes at maximum.