There are a lot to gain by studying a topic in psychology from more than one perspective.

Psychology is a science, even if it is a new science as it received a formal recognition in the nineteen century with the work of scientists interested by the study of the human psyche. Before them, we can find a long tradition of philosophers who also dealt with the same subjects but did not do it within the same scientific perspective. Psychology is born from the encounter between philosophy and science.
Psychology (from Greek psukhê, soul, and logos, word) can be defined as the scientific study of psychic phenomena, empirical knowledge or intuitive feelings, ideas, behaviors of individuals and their family, all ways of thinking, feeling and acting that characterize a person, animal or a group. It aims to investigate the psyche in terms of structure and function. It therefore seeks to describe, assess and explain the mental processes as a whole taking into account expressions of subjectivity.

The topic is hard to approach as particularly complex; the research is active and the debates intense. Certainties are difficult to obtain except in some very limited area as we shall see such as in biological psychology. And even there, large areas remain unknown. For example only 5 % of the genome located in the DNA can presently been explained.
To acquire more understanding of the psychic processes, it is then important to have different perspectives each of them having different claims and methods. It is the way that any science makes progress. Some of them will be totally or partially abandoned. Others will be confirmed by evidence. Each output of each perspective could be considered as bricks of a house called knowledge.

How does this studying process work and how can we appraise a perspective? This is the core activity of the epistemology or the theory of knowledge that Jean Piaget defines “as the study of the formation of valid knowledge”. This leads us to the questions: What knowledge (the epistemological question)? How is it made or created (the methodological question)? How to appreciate its value or validity? It is the reason why a lot has to be gained by studying a topic in psychology from more than one perspective as it throws different lights on the same topic. These perspectives can be complementary, conflicting or simply coexisting but they always bring something towards a deepest understanding of the topic. In this regard, researches on languages and meanings and on sex and gander can provide us with useful examples.

Language and meanings have been studied from three different psychological perspectives: the evolutionary, the cognitive and the social approaches.

For the evolutionary theorists, language has been developed by human beings because it confers an advantage in terms of reproduction and adaptation to the environment. “It allows knowledge and values to be transmitted so that, as well as working with others, individuals can benefit from others' experiences and perspectives.” This development was made possible because of the existence of a genetic basis.

The cognitive perspective “focus on how human beings understand language: how they process what they hear or read on order to extract meaning. The cognitive approach proposes that incoming information is combined with stored knowledge to construct a mental representation of the meaning of a piece of text or speech.”

“In contrast, the social psychological perspective proposes that our human world is created through language…Individuals use language to take action and accomplish goals, and that understanding language and creating meaning in it is about using language to do things”.
What is gained from this three above-mentioned approach?
First, they are complementary: The evolutionary perspective adopts a historical and more scientific approach based on observations.
The cognitive theories analyze the internal processes including memory leading to constitution of meaning which in turns allows individual to connect with other people; language is only a mean of expression and connection in an iterative in-out process between the individual and his environment. The cognitive theorists “focus on how language is processed by an individual in terms of lexical, syntactic and semantic information.”

The social-constructionists go further as they observe what happens when utterance is made. As for them the world is created by language, if we communicate it is because we want to do something. In this perspective, if we study discourses, we will understand the goals of the speaker and we can interpret the particular meaning by those for whom the communication is intended. Language is a resource by which meaning is co-created and negotiated between individual and groups and by which ourselves, our actions and our knowledge of the world are constructed.

Even if complementary, these three accounts can conflict: For example, “according to the cognitive perspective, meaning to some extent precedes language” while for social-constructionists, meanings come out of negotiation. But they still bring interesting claims on a complex topic because for the time being there is not a single theory that can explain such a complex subject.
Other lessons on what can be gained from studying a topic through different perspectives are given by the studies on sex and gender. Here again, four different psychological approaches can be found, each of them presenting strengths and weaknesses: the biological approaches, the evolutionary psychology, the social constructionist and the psycho analytical psychology. Each of these perspectives can also be seen as complementary.

The biological perspective defines sex and gender of people by their physical and observable differences. They looks for scientific evidence defining sex and gender such as differences in hormones, in genes or in brain; for them, biological differences between sexes can explain differences in behaviors between male and female. Even if this approach is facing some limits as it does not provide us for example a psychology of sex and gender or it does not consider the influence of culture and environment, it gives us some insights on biological processes at stake especially for solving some biological sexual problems.
As for the language and meaning, the evolutionary psychology emphasizes the importance of natural and sexual selection in the human evolution. They study the differences of behaviors between males and females for transmitting their genes and how evolution might have shaped human behavior and thinking. This approach is interesting for understanding compulsive behaviors developed by our ancestors but still used nowadays even if not adapted any more to the context.
Therefore these behaviors which were proven as being efficient have been internalized by human beings and could still be used unconsciously.
The social constructionists have a complementary approach as they explain how knowledge about sex and gender has been constructed within particular historical and social contexts and this aspect was not taken into consideration by the two above-mentioned approaches. They “take into account the historical and cultural situation of human beings and their unique capacity in animal world for meaning-making and for communication of intention”. For them, language is used for constructing the world and each person identifies him (her) self as belonging to the female or the male category as a part of an identity construction. The categories are defined by the societal values and their own understanding of these values. “Masculinity and femininity are socially and culturally constructed dimensions that inform all the forms of our thinking, emotional experience and behavior”. The weakness of this approach is to over-emphasize the role of external influence.

The psychoanalytic perspective adopts a different approach. As for social constructionists, they argue that sexuality and gender play a crucial role in the development of identity and of the self. They study essentially the process by which the meanings become internalized so as the person acquires a sense of the self. This theory were first developed by Freud but then have been criticized as it was too marked by the culture and the value of this time. The theory was enriched by further studies and the process of internalization of object and relations is studied now also after childhood. This perspective copes with the question of “becoming”: “How the baby, sexed by its biology and gendered by the society, takes on a psychological gender that becomes intrinsic to its later identity.”
As we can notice, any psychological topic is complex and it is the reason why it is important to approach them with different perspectives, none of them being able to bring a comprehensive explanation. As we can see form the two above-mentioned topics, language and meanings or sex and gender, each perspective provides a part of the explanation and theories are often complementary. Furthermore, within the same approaches, conflicts can exist as with any scientific approach and a claim can be challenged by new way of thinking. But conflicts have a positive aspect; they allow science and especially psychology, this young science, to evolve toward a better understanding of human beings' behaviors.

References
T. Cooper and H. Kaye (2002) Language and leaning, in T. Cooper and I. Roth (Eds.) Challenging psychological issues (2nd Ed, pp 71-123). Milton Keynes: The Open University.
W. Hollway, T. Cooper, A. Johnson and R. Stevens (2002) The psychology of sex and gender, in T. Cooper and I. Roth (Eds.) Challenging psychological issues (2nd Ed, pp 71-123). Milton Keynes: The Open University