The presumed rise in the incumbency effect in recent decades was really the emergence of greater electoral success for Republican incumbents. While that evidence is clear, there is one further indicator to consider in greater detail. The retirement slump indicator has a clear trend over the last century, with higher scores recorded in the second half of the century. While the problems with this indicator were discussed earlier – the indicator also reflects changes from open-seat to open-seat contests – the trend of the indicator is still likely to be cited by some as an indication that incumbents are doing better.

Not only does the indicator rise over time, but the rise is essentially the same for both parties. Figure A.1 shows the average for all existing incumbents along with the results separated by party. The trend is clearly upward over time. The issue is why, if there has not really been a significant change in incumbent fortunes.

While the rise is in part a product of the greater shift from open-seat to open-seat outcomes, discussed in Chapter 5, there has also been a clear change in outcomes after an incumbent leaves. Figure A.2 repeats from prior analyses the average change in partisan votes after incumbents left office by year for 1904–2006. The results are presented in terms of Democratic percentages, so the differences in shift by the party of the incumbent can be tracked.