Final answer:
The reason that does NOT contribute to the decline in the onset of puberty is hormonal changes in the body, as these changes result from puberty rather than cause its early onset. Nutrition, genetics, and environmental factors are actual contributors to the trend of decreasing age at the onset of puberty (option 1).
Step-by-step explanation:
The proposed reasons for the decline in the age that puberty begins include hormonal changes in the body, environmental factors, genetic factors, and improved nutrition and healthcare. However, the option that does NOT contribute to the decline in the onset of puberty is 'Hormonal changes in the body' because hormonal changes are a result of puberty rather than a reason for its earlier onset.
Nutrition has played a significant role in this decline, with historical data showing a decrease in the average age of menarche from about 17 years in 1860 to approximately 12.75 years by 1960. This change has been linked to improved nutrition and consistent access to food, which affects levels of stored body fat and the secretion of hormones like leptin that can influence the timing of puberty. Additionally, genetic and environmental factors, such as exposure to certain chemicals and psychological stress, can also affect when puberty begins.
Hence, the answer is option 1.