Following numerous investigations into human behaviour between 1939 and 1943 Maslow came to the conclusion that everyone has a set of five needs that he or she strives to satisfy. He believed that these could be ranked according to their level of importance and order in which they must be satisfied, as follows (working upwards from the bottom to the top of the pyramid): Physiological / Basic Needs – needs for physical survival eg food, water, warmth, sleep; Safety / Security – the need to feel free from physical and psychological danger; Social / Affiliation – the need to belong, to be accepted and loved by others; Esteem / Ego – the need for self respect and to be respected by others and have achievement recognised, ie being successful in a job, having ‘status’ in a group; Self-actualisation / Self-fulfilment – the need to fulfil one’s potential, ie to fully develop and maximise the use of one’s skills, creativity and ability. Maslow stated that at any one time one particular need is dominant ie needs to be satisfied. At this moment in time this need is a motivator ie the driving force that causes an individual to take action in order to satisfy the need. Once satisfied this need is no longer a motivator, but is replaced by a higher order need until this is satisfied and so on. Consequently, Maslow stated that people are only motivated by the needs that are not met and that only when our lower order needs are satisfied can people move up the hierarchy towards self-actualisation. Later research concurred with this but there was also other research that did not support his theory, suggesting that hierarchy does not exist beyond safety and security levels.