In the United States, as well as several other representational democracies, being registered as a party voter comes with some certain perks, even though it’s not required.
Primary elections, the elections that happen prior to general elections, are usually inter-party affairs, where only those who have been registered as members of a party have a particular say. It’s only once you get to the general election that party affiliation doesn’t determine what elections you can vote in.
As recently as 2004, according to NPR, political parties in the USA were fairly evenly split into thirds. However, politics have shifted pretty significantly. By 2014, according to a Pew Research Center poll, quote:
39% Americans identify as independents, 32% as Democrats and 23% as Republicans. This is the highest percentage of independents in more than 75 years of public opinion polling.
The choice to be involved in politics — or even register as a party voter — is often dependent on more than just personal choice. Canada’s statistics agency, for example, reported:
While 7% of people aged 25 to 64 whose personal income was $80,000 or over were members or participants in a political party or group, this was the case for 3% of those whose personal income was under $40,000. Viewed from a different angle, 34% of members of a political party or group had a personal income of $80,000 or over, compared with 20% of those who were not part of such a group.
And there are certainly shifts based at least partially in age. Quoting from that same NPR article, quote:
It’s a long-running pattern to see younger voters of any generation not identify with political parties. “Younger people tend to be less likely to affiliate with parties than older people,” said Jocelyn Kiley, a researcher with the Pew Research Center. But “this is as pronounced as it’s ever been.” Millennials are shunning political parties at an even greater rate than previous generations did, in part due to political dysfunction. “People give some of the most negative ratings of either party that we’ve seen in the last 20 years,” said Kiley.