Fashion
In the mid-eighteenth century, colonial Americans were able to purchase large amounts of new consumer goods, especially fashionable items like tea and cloth, that were being imported from Great Britain. They took part in what historians have described as a consumer revolution, in which greater quantities of goods were being manufactured in England and sold there and in Britain's colonies, often at prices that made it possible for even poorer colonists to participate in the new world of goods. Displaying such items helped identify one's status, or in some cases, one's aspirations to claim a higher status.
Thus, when colonists with disposable income turned to artists to paint their portraits, it is not surprising that many wanted to be presented as fashion-savvy consumers of tasteful British goods. The era was one of Anglicization, when colonists admired and sought to imitate the cultural and material practices of men and women living in England. In his portraits of elite colonists, John Singleton Copley typically celebrated their status with lush depictions of fashionable dress and elegant surroundings of rich curtains and fine furniture. Often, their poses mimicked those in portraits of British aristocrats. Some contained styles, like the turban worn by Elizabeth Murray, that were never embraced in America. In England for a time, wealthy women wore versions of turbans to masquerade balls, a practice their sisters in the colonies did not adopt. Click on Elizabeth Murray's turban for more information.
For more on the consumer revolution and colonists' purchasing habits, see T. H. Breen, Marketplace of Revolution: how consumer politics shaped American independence. (New York: Oxford University Press, 2004).