Final answer:
The belief that most lifespan developmental experts hold is that nature and nurture both interact to influence development.
Step-by-step explanation:
The belief that most lifespan developmental experts hold is that nature and nurture both interact to influence development. Providing early intervention to a six-month old baby with Down Syndrome is an example of this belief. While genetics and biology (nature) play a role in the development of Down Syndrome, the baby's environment and the interventions provided (nurture) can also have a significant impact on their development.