Weight gain is correlated with hours of sleep each night, and correlated with hours of "screen time" each day.

Weight gain is inversely proportional/ negatively correlated with the hours of sleep each night, and is directly proportional/ positively correlated with the hours of "screen time" each day. 

According to Susan Zafarlotfi, PhD, clinical director of the Institute for Sleep and Wake Disorders at Hackensack University Medical Center in New Jersey, sleep deprivation (lack of sleep) can increase a person's tendency to seek for junk foods and comfort foods and can cause a person's metabolism to function improperly. However, studies also show that getting more than the necessary amount of sleep does nothing to help a person's weight loss journey either (source: https://www.nhs.uk/news/obesity/sleep-affects-weight-loss/). So, although weight gain and sleep are negatively correlated, there must be a limit to this, as getting too much sleep does nothing to help with one's metabolism and body functions either. 

Screen time refers to the time a person spends using a device such as a computer, television, games console, etc. Studies have shown that the more screen time a person has, the greater the possibility of him/her gaining weight because it makes it easier for a person to (mindlessly/absentmindedly) snack and decreases a person's physical activity (exercise). Depending on a person's metabolism, absentmindedly snacking and lack of physical activity are definitely factors which can bring about weight gain. 

The changes to agriculture in the united states since 1900 is that increased mechanization has increased food production, thereby increasing the need for labor. there has been an increased need for higher speeds of food production due the fact that the population in the united states continues to rise.