Consumer Behavior and Decision Making Process
To understand the mental processes used in purchasing decisions
some authors employ the concept of the a figurative term used to describe the cognitive and affective processes used by a consumer during a purchase decision. The decision model situates the in a broader environment which shows the interaction of external and internal stimuli (e.g.consumer characteristics, situational factors, marketing influences and environmental factors) as well as consumer responses. The model is related to the theory of behaviourism, where the focus extends beyond processes occurring inside the consumer, and also includes the relation between the stimuli and the consumer's response.

The decision model assumes that purchase decisions occur in real time. and affected by other stimuli, including external environmental stimuli and the consumer's momentary situation. The elements of the model include: interpersonal stimuli (between people) or intrapersonal stimuli (within people), environmental stimuli and marketing stimuli. Marketing stimuli include actions planned and carried out by companies, whereas environmental stimuli include actions or events occurring in the wider operating environment and include social factors, economic, political and cultural dimensions. In addition, the buyer's includes buyer characteristics and the decision process, which influence the buyer's responses.
The purchase of up-market perfumes, often bought as gifts, are high involvement decisions because the gift symbolises the relationship between the giver and the intended recipient. The model considers the buyer's response as a result of a conscious, rational decision process, in which it is assumed that the buyer has recognized a problem, and seeks to solve it through a commercial purchase. In practice some purchase decisions, such as those made routinely or habitually, are not driven by a strong sense of problem-solving. Such decisions are termed low-involvement and are characterized by relatively low levels of information activities.
In contrast, high involvement decisions require investment of time and effort in the evaluation process. Low involvement products are typically those that carry low levels of economic or psycho-social risk. High involvement products are those that carry higher levels of risk and are often expensive, infrequent purchases. Regardless of whether the consumer faces a high or low involvement purchase, he or she needs to work through a number of distinct stages of a decision process.
The consumer buying process is usually depicted as consisting of 5 distinct stages. In the 1940s and 50s, marketing was dominated by the so-called classical schools of thought which were highly descriptive and relied heavily on case study approaches with only occasional use of interview methods. At the end of the 1950s, two important reports criticized marketing for its lack of methodological rigor, especially the failure to adopt mathematically-oriented behavioural science research methods. The stage was set for marketing to become more inter-disciplinary by adopting a consumer-behaviourist perspective.

From the 1950s, marketing began to shift is reliance away from economics and towards other disciplines, notably the behavioural sciences, including sociology, anthropology and clinical psychology. This resulted in a new emphasis on the customer as a unit of analysis. As a result, new substantive knowledge was added to the marketing discipline - including such ideas as opinion leadership, reference groups and brand loyalty. Market segmentation, especially demographic segmentation based on socioeconomic status (SES) index and household life-cycle, also became fashionable. With the addition of consumer behaviour, the marketing discipline exhibited increasing scientific sophistication with respect to theory development and testing procedures.

In its early years, consumer behaviour was heavily influenced by motivation research

which had increased the understanding of customers, and had been used extensively by consultants in the advertising industry and also within the discipline of psychology in the 1920s, '30s and '40s. By the 1950s, marketing began to adopt techniques used by motivation researchers including depth interviews, projective techniques, thematic apperception tests and a range of qualitative and quantitative research methods. More recently, scholars have added a new set of tools including: ethnography, photo-elicitation techniques and phenomenological interviewing. Today, consumer behaviour (or CB as it is affectionately known) is regarded as an important sub-discipline within marketing and is included as a unit of study in almost all undergraduate marketing programs.

For most purchase decisions, each of the decision roles must be performed

not always by the same individual. For example, in the case of family making a decision about a dining-out venue, the father or mother may initiate the process by intimating that he/she is too tired to cook, the children are important influencers in the overall purchase decision, but both parents may act as joint deciders performing a gate-keeping role by vetoing unacceptable alternatives and encouraging more acceptable alternatives. The importance of children as influencers in a wide range of purchase contexts should never be underestimated and the phenomenon is known as pester power.

Reference: https://en.wikipedia.org/wiki/Consumer_behaviour