In the article “levels of analysis revisited”, MacDougall-Shackleton discusses why it is important in differentiating between the levels of analysis and levels of reductionism, specifically ultimate functions and proximate mechanisms, when constructing research questions and forming hypotheses. This is very important when dealing with debates in evolutionary psychology because if there is no clear distinction between the two levels of analysis, is it likely that counterfactual arguments can be formed. To conclude this essay, I will discuss why I agree with McDougall-Shakleton on the necessity of clearly differentiating between the levels of analysis and why a thorough comprehension of both cause and function is required to understand behvaiour.To illustrate the instances where the levels of analysis differ, he considers the use of songbirds as examples for specific conceptual differences. At multiple points in the article, he reiterates that neither ultimate or proximate explanations are superior to the other and are not necessarily mutually exclusive alternatives. Firstly, MacDougall-Shakleton assesses a number of concepts that deal with levels of analysis and seeks to explain the differences between them. According to MacDougall-Shakleton, the term levels of analysis is also often cited as levels of reductionism. He distinguish the difference between ultimate and proximate factors by stating that ultimate factors are the variables that are responsible for determining likelihood of survival of the offspring and reproductive success of the individual and it’s offspring. Ultimate explanation is essentially the “why” an organism behaves the way it does, which includes adaptive functions or the evolutionary history of the organism. On the other hand, proximate factors are those that explain biological function in terms of immediate physiological or environmental factors. While ultimate explanation is the “why” an organism behaves the way it does, proximate explanation is considered the “how” an organism is able to do what it does. Proximate explanation includes the mechanistic causations and development of the organism.He raises a valuable argument that if researchers from either fields would accept that informations and findings from one level could possibly further research on explanations of human behavior in the other level as well. Emphasizing the importance of generating clear and concise hypotheses and weighing between alternatives, MacDougall-Shakleton(2011) states that “Confusion over falsely competing hypotheses is not limited to hypothesis at different levels of analysis.”(p.2078) and that it can happen with researchers who are in the same field that are creating hypotheses that examine similar interpretations. He cites Dennett who defines the term ‘greedy reductionism’ as the appreciation of reductionism of lower levels and discounting of higher levels of reductionism (Dennett, 1995 as cited by MacDougall-Shakleton, 2011) but coined his own term to refer to this attitude as ‘reductionist snobbery’. However, to completely understand behaviour, especially human behaviour we need to evidence from both proximate and ultimate levels of analysis. It is possible that information from a proximate level can help with contributing to ultimate level hypotheses and it has been debated that without proper integration of the two levels of analysis it could lead to inaccurate perceptions in the resulting hypotheses. Although, it has also been counter-argued that integration across the levels of analysis is not a good strategy as a basis for research. From an adaptationist point of view, MacDougall-Shakleton suggests that the assumption that natural selection shapes cognitive and neurological mechanisms, it has helps determine the molecular, cellular or system level processes. He also claims that it is essential to note that concluding generating and analyzing a hypothesis by way of adaptive function is not the same as acknowledging that the hypothesis is correct. Furthermore, using the integrative approach to analyze and possibly dismiss the hypothesis, considerations of function and evolutionary history is needed to develop the aforementioned hypothesis. Consideration of adaptive function provides motivation and direction to explore novel proximate mechanisms. It is also possible that consideration of adaptive function can develop into proximate hypotheses that are inaccurate and/or not supported. However, if a hypothesis was based on ideas from the ultimate level of analysis, it is unrelated to if the hypothesis about proximate mechanisms that was derived from evolutionary predictions is supported by the data or not. Lastly, MacDougall-Shakleton emphasizes the various uses of the levels of analysis, and explored the benefits of using an integrative approach that uses an understanding of adaptive function to guide exploration of proximate mechanisms. It is essential to recognize that the use of an integrative approach is not without uncertainty and to thoroughly assess the mutual exclusivity of the two hypotheses. Two hypotheses may be non-exclusive even if they are at exactly the same level of analysis and level of reductionism. Much false debate could be avoided by careful consideration of whether or not alternative hypotheses are or are not mutually exclusive alternatives. Because ultimate and proximate levels do not directly compete, however, does not mean they should have nothing to do with each other–In my evaluation of MacDougall-Shakleton’s article, I will be explaining why I agree with his views that explanations of both cause and function are necessary in order to have a complete understanding of behaviour, especially human behaviour, which is usually conducted using an integrative approach. The separation of classifying proximate from ultimate causation presumably leads to better understandings of the events and systems concerned. Consideration of function can guide research into mechanisms in many ways. Similarly, research into adaptive function can be stimulated and guided by a detailed appreciation of mechanisms. Although there is risk of confusion, careful consideration of one level of analysis can benefit research at the other.