Sexual intercourse is a biological imperative for all animals, including humans, as it guarantees the survival of the species by transmitting favorable genes to the next generation. However, human sexuality is much more complex than simple reproduction. Humans are rational, social beings with emotions, beliefs, and behaviors influenced by both biological and cultural factors. Sexual behaviors are caught in this web of complexity, making human relationships extremely complicated and easily influenced by internal and external forces and motivations. For example, cultural assumptions about sexual desire, or libido, between males and females have become almost exclusively cultural phenomena. While there is a biological basis for these gendered representations of sexual desire, it is not always as clear as it seems. When someone is hungry, there is an internal force that tells them to eat, and things like the smell or even the sight of food can influence that hunger. Sexual desire and hunger are basic instincts that humans are biologically programmed to have and can both be influenced by external forces that stimulate the senses. Additionally, psychological factors such as ghosts from the past, interruptions, or a tiring day at work can arouse unconscious feelings and influence a person's sexual desire (Small 44). Along with individual differences, the complexity of the human brain makes it difficult to determine which stimuli instigate sexual arousal. According to the textbook Our Sexuality, “we don't all respond the same way to the same stimuli. Some people can become very aroused if their partners use explicit sexual language; others find such words threatening or a sexual turn” (Crooks and Baur 146). Each person lives a unique life, so it stands to reason that the same behavior can be pleasant or disgusting depending on who is being asked. Although individual differences play a significant role in sexual arousal and desire, there are specific parts of the brain involved in sexual thoughts and behaviors for all genders. Crooks and Baur mention some. The cerebral cortex, responsible for higher mental processes, is the place of sexual fantasies.