What are the principles that define the Biological level of analysis ?

There are 3 principles that define the Biological level of analysis , and each come with a research study conducted by researchers :Principle 1 : Biological correlates of behaviour - looks at the physiological origins of behaviour , eg. studies the effect and role of neurotransmitters / hormones / genes on the behaviour ----- study : " Newcomer et al. - Studies the role Cortisol (stress hormone) on verbal memory "Principle 2 : Animal Research is used to understand certain facts about human behaviour : looks at neuro plasticity ----- study : "Rosenzweig and Bennet "- Studies the effect of either and enriched or poor environment on the neuroplasticity of rats' brainsPrinciple 3 : Human behaviour can also be genetically based - This principle highlights that GENES play an important role in explaining behaviour, however it rarely presents a full explanation, as behaviour is affected both by genes and environmental factors ------- study : " Bouchard et al. - studies the concordance rate in IQ in monozygotic twins (share 100% of genetic material) who either raised apart or together. This study's results demonstrate that environmental factors play an important role in shaping IQ and behaviour.