Choose one of Baltes' key principles of life-span development and provide both a definition and an example of the principle.

One of Baltes' key principles of life-span development is the principle of multidirectionality. This principle refers to the idea that development involves both growth and decline, and that these changes can occur simultaneously in different areas of life.

For example, as individuals age, they may experience physical decline in terms of strength and flexibility, while also experiencing growth in their knowledge and wisdom. This principle recognizes that development is not always linear and can involve both positive and negative changes at the same time.