Final answer:
Marcus's struggles with anxiety and depression in adulthood are related to the concept of neuroticism, a personality trait linked to negative emotions. Studies have found that neuroticism may have genetic roots and be influenced by early life experiences, both potentially leading to long-term mental health challenges.
Step-by-step explanation:
The concept of neuroticism is central to understanding Marcus's situation, as it refers to a personality trait associated with experiencing negative emotions, such as anxiety and depression. Throughout various studies, such as those by Friedman and Booth-Kewley and Watson and Clark, there has been a clear linkage between neuroticism and mental health issues. Notably, neuroticism can be both a result of genetic predispositions and early life experiences. Babies, like Marcus, showing signs of sensitivity and anxiety, may be expressing an inherent temperament, which can later influence their proneness to mental health issues if not mitigated by a supportive environment.
Additionally, Moffitt et al.'s research on childhood risk factors associated with mental disorders suggests that individuals with high levels of neuroticism are at a greater risk for generalized anxiety disorder and depression later in life. Moreover, the study on rat pups by Raineki, Cortés, Belnoue, and Sullivan supports that early life experiences can result in changed behavior and brain function, further emphasizing the role of environment alongside genetic factors. It is essential to consider both biology and personal experiences when examining an individual's mental health trajectory.