1. Developmental psychologists study development to understand human nature across the lifespan. They carry out developmental research to better understand the mechanisms that produce human behavior such as: environmental influences, genetic influences and interactions with others. This information is accessible to a wide range of people, including: academics, theorists, parents, educators, medical and mental health professionals, and those who wish to better understand themselves and others. Developmental psychologists study: physical and psychomotor development (sensory systems, body control), cognitive development (perception, memory) and socio-emotional development (self-esteem, personality). These developments can occur either qualitatively. An example of qualitative development is Piaget's theory of cognitive development which details how humans develop through the stages of understanding the world through their interactions with physical objects. Children under 6 months of age lack object permanence; they believe that an object no longer exists when it is no longer in sight. However, around the age of 8 months, the child understands that an object that is covered or no longer visible still exists. An example of quantitative development includes physical growth over time, such as a child's weight or height.2. The four main questions and themes in developmental psychology that we discussed in class include: stability and plasticity of human behavior, continuity versus discontinuity, normative versus ideographic approaches, and nature versus education. The stability of human nature refers to the idea that things remain constant over time. For most of the 20th century, scientists believed that development remained constant over time, so much so that they believed that experiences early in life played a critical and irreversible role in development.