Final answer:
Improved sanitation, hygiene, and access to clean water were the main causes of the increase in lifespan before the 1950s. The development of immunizations and antibiotics also played a significant role.
Step-by-step explanation:
The increase in lifespan before the 1950s was primarily caused by two factors: improved sanitation and hygiene, and access to clean water. These factors helped prevent the transmission of many diseases. Early in the 20th century, people learned the importance of boiling bottles before using them for food storage and baby's milk, washing their hands, and protecting food from flies.
In addition, the development of immunizations for various diseases and the discovery of antibiotics, such as penicillin, in 1941, further contributed to controlling infectious diseases and extending lives.