One assumption of the behaviourist approach is that we are born as a blank slate (known as tabula rasa) and we learn everything from the environment. This assumptions believes that all humans, male and female, are born the same and it is the environment and social norms that shape our personality. With gender roles, behaviourism believes that men and women are born the same but it is the roles built into the environment that create the stereotypes that we see, for example certain toys bought for girls and boys and colour differences pink and blue.
Another assumption of the behaviourist approach is that we learn through conditioning. There are two main types of conditioning these are classical conditioning (also known as pavlovian conditioning) and operant conditioning. Classical conditioning refers to learning through association. For example in Pavlov's work with dogs they learned that the sound of a bell ringing was associated with gaining food, and therefore in the future salivated whenever they heard a bell ring. Whereas Operant conditioning refers to learning through reinforcement. These are usually split into positive reinforcement (making your more likely to re-enact a behaviour), negative reinforcement (also making you more likely to re-enact a behaviour) and punishment (making you less likely to continue a behaviour). For example in Skinner's work with rats they were given positive reinforcements in the form of food if they pressed a lever, and as soon as they learned this they continued to press the lever for more.