Final answer:
Frank and Lillian Gilbreth developed the principle of motion economy, which involves breaking down jobs into elementary motions to increase efficiency and productivity.
Step-by-step explanation:
The principle developed by Frank and Lillian Gilbreth is motion economy.
They believed that every job could be broken down into a series of elementary motions, and by reducing the number of motions required to perform a task, workers could become more efficient and productive.
Frank and Lillian Gilbreth used time and motion studies to find ways to increase productivity and applied their methods not just to industry, but also to the home, office, and other areas.