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Lead

Lead paint is a scary topic, especially in Massachusetts where a lot of the housing is old. Lead paint is also a complex topic. Because the effects of lead paint on children and adults are dire and since many children have suffered permanent disability because of exposure to lead paint, state and federal governments take a very proactive and cautious stance regarding lead paint. The authoritative sources are your state and federal websites dedicated to lead paint issues (some links are below).
Apparently lead was added to paint because it enhanced its longevity and was government sanctioned. This same website states that the dangers of lead paint began to be realized in the 30s and 40s and in 1955 paints that were to be used in home interiors were restricted to less than 1% by weight of lead. Lead paint was also expensive and used mostly for living rooms and window sills and other locations where durability and quality were important.
From the EPA and state materials (which, again, are the materials you should refer to) it seems that the biggest risk is from chipping and peeling paint which can be eaten by children (they like the sweet taste) and dust from lead paint released from windows and door jambs. If paint is in good condition it is of lower risk and can be painted over and encapsulated as far as I could understand. Again, this is a complex, thorny topic with the health of children at stake, so even though it takes time, please go through the official documentation.
A common question is trying to judge the risk of lead paint based on the age of the house. The most common cut-off is 1978. Houses built before 1978 are assumed to have lead paint and there are some rules that govern the purchase and sale of such houses. Before 1955 paints contained as much as 50% by weight of lead. This was reduced to 1% by weight in 1971 (by the Feds) and then, since 1977, the limit is 0.06% by weight. Massachusetts uses 1950 as a criterion for 'old' houses as far as lead goes. This EPA page has a rough graph showing the percentage of homes having lead paint by year of construction.
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