Very few people really talk about maternal instinct in the biological sense. It is mostly used in more general conversations and hence it is part of our cultural language. However when people talk about the sexual instinct then they talk biologically. An instinct suggest a biological matter.
But then again, when people talk about maternal instinct they talk about a feeling. A feeling they have when they are having a baby. Or a feeling they think they will have. A feeling they think most mothers have. Or should have. Or ought to have if they are good mothers. In most cases people tend to talk about maternal instinct in an unconscious way. There is no thorough thought process. The words are part of a cultural language, like we have associations about religion or minority groups. The words people use for these also have a meaning that changes throughout time and place, in tune with changing societies. But in a specific time period and place, people do not ask whether maternal instinct is biologically or culturally determined. People assume they know the (absolute) meaning.
Most of the time people do not even think about the meaning of maternal instinct. Maternal instinct was most likely explained to them a long time ago or maybe not even that. People can assume the meaning because of the separate words and therefore assume existence. The meaning is very much linked to their own beliefs, their cultural language and their cultural definition of motherhood? Maternal instinct is redefined when the cultural associations change and most of the time this takes a generation or two.
When people of the women’s liberation movement talk about maternal instinct, most of the time, they refer to the biological standpoint. Most of them (especially during second wave feminism between the 1960′ and the 1980’s) believe that maternal instinct is created and or invented by male scientists and so they often discard the idea of maternal instinct. They can be profoundly critical of the very idea of maternal instinct but then they are talking about a different meaning of the word. The literal sense, the proven scientific sense. They will say that its existence has been proven in the laboratory rat through hormone injections but not with humans. They will take it very literal and they will look for proof and statistical evidence. They are making their way to the oppressive nature of motherhood.
Other people will wave these arguments away. They will insists on the fact that they do know what they are feeling when their children are not with them.
But in point of fact, scientists today analyze maternal instinct in a biological, purely hormonal sense and scalp all feelings and emotions away. And maternal instinct have not scientifically been proven by scientists after the lactative period. It simply has not.