We can define an instinct in terms of two things:
Firstly, it is behaviour that is genetically-programmed from birth. An instinct is not something that we could choose not to do. This is significant, in terms of human behaviour because it suggests that if people have instincts that are literally forced to follow certain pre-programmed courses of action to the exclusion of other possible courses. We can illustrate this idea with and example from the animal world
Birds have a nest-building instinct. At a particular time in the year, a bird (such as a blue tit) instinctively starts to build a nest. It is not something that an individual blue tit can choose not to do because it is compelled by its genetic programming to do this.
Secondly, an instinct doesn't only tell us what to do at a certain time, but, equally-importantly, it tells us how to do it. This is significant because instinctive behaviour does not, by definition, have to be taught.
Thus, to continue the above example, a blue tit is not only forced to build a nest; instinct also tells it how to build a certain type of nest. Once again, the bird has no choice in the matter.
It cannot, for example, decide that this year, to be different and because its grown tired of building the same old nest year after year, it will build a three-story super nest with a private bedchamber and en-suite bathroom...
These ideas are significant because they add a further dimension to our understanding of the question of whether or not human behaviour has an instinctive basis. For the moment, considered in these very basic terms, it would appear difficult in the extreme to state unequivocally that human behaviour is instinct-driven. We are led to this conclusion because the complexity of human social behaviour suggests that simple instinctive behaviour does not explain the huge range of behavioural choices made by people of different cultures and different times.