I thought the opposite was true, given the number of famous start-ups that have been co-founded.
From a new published paper by Cooper & Saral (European Economic Review, January 2013).
Entrepreneurs are surprisingly unlikely to have partners. One possible explanation for this is that entrepreneurs have distinctive preferences for working alone rather than in teams. However, a number of alternative explanations exist such as an inability to locate suitable partners or low profitability from having a partner. Utilizing a diverse subject population with a high proportion of active entrepreneurs, we use a team production experiment to directly examine whether entrepreneurs prefer to work alone or in a team. The experiment also measures an important determinant of entrepreneurs' performance within teams, their relative tendency to free-ride. The data indicate that entrepreneurs, while no more likely to free-ride on their teammates, are substantially more interested in working alone than similar non-entrepreneurs.
The title of the paper is "Entrepreneurship and Team Participation: An Experimental Study," and a draft of the paper (February, 2010) is here.