For divorce purposes, Massachusetts gives very broad definition to "marital assets". Absent a pre-nuptial agreement every asset and liability owed by either party is considered by the Court in a division. The Court can consider the source of the asset but that is just one of the factors in whether or not to divide it. Under M.G.L. ch. 208 § 34, the Court can assign alimony or division of property based on the following section:
"In determining the amount of alimony, if any, to be paid, or in fixing the nature and value of the property, if any, to be so assigned, the court, after hearing the witnesses, if any, of each party, shall consider the length of the marriage, the conduct of the parties during the marriage, the age, health, station, occupation, amount and sources of income, vocational skills, employability, estate, liabilities and needs of each of the parties and the opportunity of each for future acquisition of capital assets and income. In fixing the nature and value of the property to be so assigned, the court shall also consider the present and future needs of the dependent children of the marriage. The court may also consider the contribution of each of the parties in the acquisition, preservation or appreciation in value of their respective estates and the contribution of each of the parties as a homemaker to the family unit." (emphasis added)
Generally, any asset earned during the marriage will be divided 50/50. A stay at home parent's contribution as homemaker and in child-rearing is considered equal in value to the earning capacity of a working parent. A good example of this is the equal division of retirement accounts earned during the marriage.
Regarding assets earned prior to the marriage, those are more difficult to predict. The other factors, such as the length of the marriage, health of the parties, etc. can have a significant impact on whether or not these assets are divided. Generally we tell clients that in short term marriages (less than 5 years) the Court tries to put people back in the position they were in prior to the marriage (returning previously owned assets).
As marriages get longer, though, it is very typical for assets to "merge" into the fabric of the marriage. A major factor in this consideration is whether or not the asset was shared during the marriage. For instance the marital home (if purchased by one party prior to the marriage) is pretty likely to "merge" as more time goes by because both parties contribute to its "preservation and appreciation"(even if the title is not changed).
If you are concerned about keeping pre-marital assets separate you should consult with an attorney regarding the preparation of a pre-nuptial agreement prior to the marriage. If you are already married and considering a divorce, you should consult with an attorney to explain your specific circumstances before assuming property will be kept separate or merged by the Court.