This week’s move by seven Labour MPs to quit the party will no doubt evoke memories of the Labour-SDP split in 1981 which, it has often been claimed, gifted Margaret Thatcher her landslide at the next general election.

The rationale for this theory is intuitive, since the gang of four, and every subsequent defector bar one came from Labour. Moreover, the changes in the popular vote between 1979 and 1983 look a lot like they would if the only major move were a split on the left. The Conservatives lost 1.4 points, Labour lost 9.4, and the Liberal-SDP Alliance gained 11.9 points. And the national pattern was repeated in seat after seat.

This is, however, a gross oversimplification. Changes in popular vote share are the sum of a number of offsetting flows of voters in different directions. And even if we knew these flows, we still don’t know for sure how people would have voted without the SDP’s intervention. It’s often assumed that absent the split (and the Alliance surge), 1979 Labour voters would simply have stayed loyal.

Ivor Crewe and Tony King examined this in their 1996 book SDP: The Birth, Life, and Death of the Social Democratic Party, and found it wanting. Though because data they were using required them to extrapolate from the imputed second preferences of a small subset of Alliance voters, their findings are necessarily accompanied by a bit of an asterisk.

We do, however, have an alternative data source. The 1983 British Election Study asked how people had voted in that year’s election, as well as in 1979. But the BES also asked people what their second choice of party would have been.

The relevant group here is the five million people who didn’t vote for the Liberals in 1979, but did vote for the Alliance in 1983. This group did back Labour more than the Conservatives in the previous election (though not exclusively).

How might these Alliance switchers have voted in the absence of the SDP or resurgent Liberals? The BES second preferences suggest that roughly as many of them would have voted Tory (50 per cent) in 1983 as Labour (45 per cent).

If we assume that the Liberal vote had remained at 14.1 per cent and reallocate the 11.9 point Alliance gain based on second preferences, the likeliest “alternative” scenario is the Conservatives on 49.5 per cent, up 4.6 points from 1979 and Labour on 33.7, down four, or if we average the two changes, a 4.3 per cent swing. I’ve used a decimal place to avoid rounding confusion, but these numbers are still subject to assumptions and margins of error.

This swing is almost identical to the actual swing of 4 per cent. Compared with the real 1983 result, the higher Labour and Conservative vote shares and lower Liberal share would mostly have benefited the Tories in terms of seats (15 to 20 more than the 394 they got), much as was seen when the SDP/Liberal surge was finally reversed in 2015. My best guess is that Thatcher’s majority would have been closer to 180 than the actual 144.

Party 1979 result Actual 1983 result Alternative* 1983 result CON 44.9 43.5 (-1.4) 49.5 (+4.6) LAB 37.7 28.3 (-9.4) 33.7 (-4.0) LIB/SDP 14.1 26.0 (+11.9) 14.1 (0.0) OTH 3.3 2.2 (-1.1) 2.7 (-0.6) Swing 4.0 per cent LAB to CON 4.3 per cent LAB to CON *Estimate based on constant Liberal share and reallocation of Alliance switchers based on 1983 BES second preferences. Data sources: Number Cruncher, British Election Study.

What actually happened in 1983? The Liberal-SDP Alliance won similar numbers of votes from Labour and the Conservatives, but past Labour voters were also likelier to switch to the Tories than vice versa. Labour also lost out elsewhere in the churn, leading to a crushing defeat.

Of course, how much this story of the early 1980s tells us about how The Independent Group may fare is another matter entirely. But if we are going to use the past as context, it’s good to get the facts right.