There are issues with asking the mothers to be 'observers'- they are unlikely to be as objective as those trained, and may have been biased in terms of what they reported on. They may not have noticed when their baby was showing signs of anxiety, or could have misinterpreted it. Due to the social sensitivity of the research, mothers may have altered their responses to meet societal expectations of development. Therefore, even if the babies did behave naturally, it may not have been recorded accurately.