W 1985 został profesorem nauk politycznych w Europejskim Instytucie Uniwersyteckim we Florencji, stanowisko to piastował aż do przejścia na emeryturę w 1994 roku.
Ważniejsze prace Voters, parties and leaders: the social fabric of British politics. Harmondsworth: Penguin Books, 1963 ; An Introduction to Comparative Government. Londyn: Weidenfeld & Nicolson, 1969. ; Comparative legislatures. Nowy Jork: Prentice-Hall, 1973 ; Political parties. A genuine case for discontent? Londyn: Wildwood House, 1978 ; The Discipline Of Politics. Londyn: Butterworths, 1981 ; Political leadership : towards a general analysis. Londyn: SAGE, 1987 ; Cabinets in Western Europe, Basingstoke: Macmillan, 1988 ; Governing together: the extent and limits of joint decision-making in Western European cabinets, Nowy Jork: St. Martin's Press, 1993 ; Party and government: an inquiry into the relationship between governments and supporting parties in liberal democracies, Nowy Jork: St. Martin's Press, 1996 ; People and Parliament in the European Union: participation, democracy, and legitimacy, Oxford: Clarendon Press, 1998 ; The nature of party government: a comparative European perspective, Nowy Jork: St.