Forget the David Cameron autobiography or knitted jumper from Granny – we might be getting a general election for Christmas. While Brexit is going to be the obvious dividing line, elections always see broader economic and social trends coming to the fore.

The biggest of these used to be traditional class divides. In 1974, when the two main parties last both got around 40% of the vote, the Conservatives enjoyed a 37-point lead among the upper/middle class, while Labour enjoyed a 35-point lead among working-class voters. By 2017, those gaps had virtually disappeared.

So, if class is no longer the single big determinant of how people vote, what is? Age. While talk of a “youthquake” came to the fore in the wake of the 2017 election, age as a strong driver of which party we vote for has been building for decades. In the 70s, our voting patterns didn’t change much by age but by 2017 a 30-year-old was twice as likely to back Labour over the Conservatives, while a 70-year-old was twice as likely to do the opposite.

Graph showing percentage of voters by age in the 1974 and 2017 elections ‘In the 70s, our voting patterns didn’t change much by age.’

In part, these divides reflect that generational gaps have taken on more of a class tinge – those 70-year-olds have over twice the home ownership rates of the 30-year-olds. But that is exactly why age divides in politics are so worrying: they encourage parties to focus on appealing to their age-specific bases when what 21st-century Britain needs is a government focused on overcoming generational divides.

• Torsten Bell is chief executive of the Resolution Foundation. Read more at resolutionfoundation.org