I originally posted this in ELI5 but got no answers. Maybe I'll have better luck here! Many thanks. I know it's kind of basic.
Though my flair focuses on slavery and I am today a labour historian, my background is originally in Caribbean history, so I can try and offer some insight on that end of the topic. I'm afraid I don't feel confident enough with my recent Caribbean social history to comment on wider criminal trends and their origins, so this post will focus on political violence.
In part, Jamaica's unusual level of electoral violence historically can be explained by the character of its party political system, in contrast to that of Barbados. Although both island nations have a firmly entrenched two-party system, in Barbados, they share a common political heritage: the Democratic Labour Party is an off-shoot of its main rival the Barbados Labour Party, and it emerged initially as a minor third-party into an existing democratic political culture. They share a common voter base and have remarkably similar political visions to one another for the future of the island. Furthermore, it is only since the late 1960s that the Barbadian political system has coalesced around two main parties; before 1971, Barbados usually had three parties in its Parliament, all having returned MPs with considerable levels of popular support. There has never been much of an incentive for Barbados to develop intense levels of political violence in that context.
In contrast, Jamaica's two main political parties both emerged relatively quickly alongside each other, and represented two quite different constituencies. The People's National Party (PNP) emerged to represent the nationalistic and social democratic interests of the island's middle class; the Jamaica Labour Party (JLP) emerged initially as an extension of the island's main trade union and as a platform for Alexander Bustamante, who would be the country's first Prime Minister, to secure power. The JLP was initially essentially ideology-less, but quickly came to appeal to the more conservative sentiments of the island's black working class; there is also evidence that it absorbed support from the short-lived conservative Jamaica Democratic Party. Both parties were orientated around strong, charismatic leaders who led their parties with a fair degree of dominance and authoritarian leadership (although the PNP was a much more democratic and robust party from the outset - the JLP barely existed as a real party for its early years). Both parties were fierce rivals from the outset and in the earliest elections were competing over the very destiny and shape of the nation.
There appear to be underlying causes for why such intense political violence developed between the two parties. The first is that they went above and beyond the norms of democratic engagement to cultivate a political base; both parties for much of Jamaican history promised material benefit and favourable treatment for the constituencies that supported them. Rather than governing in the interests of the whole nation, both the PNP and the JLP adopted political strategies of governing in support of the interests of their own backers for much of Jamaican history. This kind of clientèle politics helped to create passionate core supporter basis in the electorate; incredibly safe, uncompetitive constituencies emerged in which most of the voters had an enormous personal interest in keeping 'their' party in power. Both social and physical violence were thus used to drive supporters of the opposition away and to keep the opposition from campaigning in these seats; this violence could naturally spill out over into marginal battleground seats because rather than seeing as the government as their provider and protector, these voters saw their party as fulfilling those functions.
The leadership of the main parties were also complicit in violence, to the point where they openly courted Jamaica's criminal underground to help build up their support base and intimidate the opposition. Criminal networks became powerful tools to disrupt opposition campaigns, intimidate wavering voters, suppress criticism and interfere with the electoral process. As a result, gang feuds and other criminal disputes could become entangled with party politics during election campaigns. In 1976, the PNP tried somewhat earnestly to crack down on political unrest, but it turned into a campaign of repression against the JLP - though it does appear that in that election, the JLP had been the main agitator of violence.
It was only after the 1980 election in which hundreds were killed that both parties recognised how desperate the political situation had become, and how untenable it was to continue in this way; both thus began to make a sincere effort to improve their behaviour and reform the law to make overtly criminal and fraudulent practices much more difficult to get away with. Invariably, political violence is connected to political outcomes in Jamaica; the 1961 referendum on the West Indies Federation was almost completely peaceful because neither side felt they had anything significant, material or cultural, to lose.
In essence then, in terms of political violence, the differing character of the dominant political parties goes a long way to understanding why Barbados has had a peaceful experience of democracy and Jamaica such a troubled one. Jamaica's parties were born bitter rivals at a time when whoever assumed office decided the fate of the nation; they both opted to pursue strategies of favouring their own voters excessively and blatantly, and creating strongholds of support for themselves in the process. As political violence developed as a consequence, they embraced rather than shied away from it until it reached truly horrendous proportions. All of this is in sharp contrast to the experience of Barbados, where the two main parties of today emerged into their positions in very different circumstances, and where the democratic culture was much more robust from the outset because both parties were initially competing for the same blocs of voters owing to their compatible ideologies and common heritage.
Amanda Sives' Elections, Violence and the Democratic Process in Jamaica 1944 –2007 is a fantastic exploration of this phenomenon, one I thoroughly recommend if you want to learn more and the basis of much of this post. In addition, I would also suggest the following articles might be of interest:
Greetings all! This is the sort of question that can attract casual answers, so just a reminder to prospective respondents of which subreddit this is: it's /r/AskHistorians, so be mindful of the rules here, e.g.
thx!