Your behaviour and your biology.

 When your biology changes so does your decision making, puberty would be one glaring example. Those drives you understood to be yours, spiteful, accommodating, loving, hateful, are affected by small changes in your biology.

What indications are there that behavior has a biological basis? 
[text provided by Joseph McInerney]

• Behavior often is species specific. A chickadee, for example, carries one sunflower seed at a time from a feeder to a nearby branch, secures the seed to the branch between its feet, pecks it open, eats the contents, and repeats the process. Finches, in contrast, stay at the feeder for long periods, opening large numbers of seeds with their thick beaks. Some mating behaviors also are species specific. Prairie chickens, native to the upper Midwest, conduct an elaborate mating ritual, a sort of line dance for birds, with spread wings and synchronized group movements. Some behaviors are so characteristic that biologists use them to help differentiate between closely related species.
  
• Behaviors often breed true. We can reproduce behaviors in successive generations of organisms. Consider the instinctive retrieval behavior of a yellow Labrador or the herding posture of a border collie.
  
  
• Behaviors change in response to alterations in biological structures or processes. For example, a brain injury can turn a polite, mild-mannered person into a foul-mouthed, aggressive boor, and we routinely modify the behavioral manifestations of mental illnesses with drugs that alter brain chemistry. More recently, geneticists have created or extinguished specific mouse behaviors—ranging from nurturing of pups to continuous circling in a strain called "twirler"— by inserting or disabling specific genes.
  
• In humans, some behaviors run in families. For example, there is a clear familial aggregation of mental illness.
  
  
• Behavior has an evolutionary history that persists across related species. Chimpanzees are our closest relatives, separated from us by a mere 2 percent difference in DNA sequence. We and they share behaviors that are characteristic of highly social primates, including nurturing, cooperation, altruism, and even some facial expressions. Genes are evolutionary glue, binding all of life in a single history that dates back some 3.5 billion years. Conserved behaviors are part of that history, which is written in the language of nature's universal information molecule—DNA.