The Social Democratic Party appeared too late to qualify as one of the original parties in Janda's 1950-1962 ICPP study. It continued to1990 and qualified for the Harmel-Janda study of party change.

The essay on party politics in the United Kingdom from 1963 to 2000 says:

In 1981 several former Labour ministers formed the Social Democrats to constitute a "responsible" Left (or center). They made an agreement with the Liberal Party for the 1983 elections and combined as the Social Democratic Alliance, winning 25% of the popular vote but (due to the first-past-the-post system) less than 5% of the seats in parliament.) Not surprisingly, the Social Democrats also advocated reform of the electoral system from single-member constituency to proportional representation. The alliance split in 1986 on the issue of nuclear weapons--the Social Democrats favored continued reliance upon nuclear weapons, and the Liberals a more antinuclear stand. The SDP disappeared in 1992 after the two remaining SDP legislators were defeated (Rasmussen and Moses, 1995: 135).

Consult the index to variables for annual scores of the party's issue orientation, organizational complexity, centralization of power, and coherence from 1950 through 1990.