Twin studies are pivotal in understanding the genetic basis of behaviour, as they allow researchers to tease apart the influences of nature (genetics) and nurture (environment). Identical twins, who share 100% of their genes, provide a unique opportunity to examine the role of genetics in behaviour. If a trait or behaviour is more common in identical twins than in non-identical twins (who share about 50% of their genes), it suggests a genetic influence. Moreover, studying identical twins reared apart offers insights into how different environments can impact individuals with the same genetic makeup. These studies have shown that many traits, including intelligence, personality, and the risk of developing certain mental health disorders, have a significant genetic component. However, twin studies also highlight the role of environmental factors in shaping these traits. By comparing similarities and differences in twins, researchers can gain a deeper understanding of how genetics and environment interact to influence human behaviour.

Evolutionary psychology explains fear and anxiety in modern humans as adaptive responses that have evolved over time. These emotions were crucial for our ancestors' survival, enabling them to detect and respond to threats in their environment. The "fight-or-flight" response, triggered by fear or anxiety, prepares the body to either confront or flee from danger. This response is mediated by the sympathetic nervous system, which increases heart rate, blood flow to muscles, and releases adrenaline for rapid action. In the modern context, while we may not face the same threats as our ancestors, the physiological response to perceived dangers remains. Evolutionary psychology suggests that anxiety disorders may represent a maladaptive expression of this fear response, where the body's natural alarm system becomes overly sensitive or misinterprets non-threatening situations as dangerous. This perspective provides a framework for understanding why certain fear and anxiety responses exist and how they might sometimes become dysfunctional in today's world.

Hormonal changes during different life stages have a significant impact on behaviour and mood. For instance, adolescence is marked by hormonal shifts due to puberty, leading to changes in mood and behaviour. Increased levels of hormones like testosterone and estrogen can result in heightened emotions, impulsivity, and risk-taking behaviours. Similarly, during pregnancy, fluctuations in hormones such as estrogen and progesterone can affect mood and emotional state, contributing to mood swings and, in some cases, prenatal depression. Menopause, characterised by a decline in estrogen and progesterone, can lead to mood swings, irritability, and increased risk of depression in some women. Furthermore, hormonal fluctuations can affect circadian rhythms, impacting sleep patterns and overall mood. These life stages illustrate how hormonal changes can exert a profound influence on psychological wellbeing, underscoring the importance of hormonal balance in mental health.

Brain injuries can have profound impacts on personality and behaviour, providing critical insights into the functions of different brain regions. Damage to specific brain areas can result in changes in personality traits, emotional regulation, and cognitive abilities. For example, injury to the frontal lobes, particularly the prefrontal cortex, can lead to disinhibition, impulsivity, and changes in social behaviour. This area is responsible for executive functions, such as decision-making, planning, and social behaviour. The famous case of Phineas Gage, who suffered a frontal lobe injury, dramatically illustrates this. Post-injury, Gage exhibited significant changes in personality, becoming impulsive and socially inappropriate, which was starkly different from his previous behaviour. Similarly, damage to the limbic system, involved in emotion and memory, can result in altered emotional responses and memory impairments. These observations from brain injury cases underscore the specialized functions of different brain regions and how damage to these areas can result in specific behavioural and personality changes, providing valuable information about brain function and its relation to behaviour and personality.

Neurotransmitter imbalances play a crucial role in mental health, particularly in conditions like depression and anxiety. Neurotransmitters are chemical messengers in the brain that transmit signals between neurons, influencing mood, thought processes, and behaviour. In depression, there is often a deficiency of neurotransmitters like serotonin and norepinephrine, which are associated with mood regulation. This deficiency can lead to symptoms like persistent sadness, lack of interest, and fatigue. Anxiety disorders are also linked to neurotransmitter imbalances, particularly involving serotonin and gamma-aminobutyric acid (GABA), a neurotransmitter that inhibits nerve transmission in the brain, creating a calming effect. Disruptions in these neurotransmitter systems can lead to heightened responses to stress, excessive worry, and other anxiety symptoms. Treatments for these conditions, such as selective serotonin reuptake inhibitors (SSRIs), work by correcting these imbalances, thereby improving symptoms. This highlights the vital role neurotransmitters play in maintaining mental health and how their dysregulation can lead to significant psychological disorders.