Most of us have participated in a conversation about which figures, living or dead, we would like to invite for dinner or whose ideas have had the greatest impact on history. According to a book called The 100, published in 1978, the Reverend T.R. Malthus is the eightieth-most-important person of all time, above both Lenin and Machiavelli. Even if they might not fancy inviting Malthus round for dinner, many historians of science would be comfortable with the suggestion that his population principle – the theory that populations increase at a much faster rate than their means of subsistence – is one of the more important ideas in history, probably because of its influence on Charles Darwin and Alfred Russel Wallace's work on evolution. Yet in 1998, the two-hundredth anniversary of the first edition of Malthus's Essay on the Principle of Population, there was little in the way of fanfare. Few historians were queuing at the doors of trade publishers with ideas for books that might inform a mass audience about Malthus's big idea. Why was that?

One reason is that Malthus is a tough sell. He was a prophet of doom: a man of the cloth who seemed to be lecturing the poor about their habits and arguing that they were the cause of society's problems. If that was not bad enough, Malthus also took what people who identify with progressive politics consider the wrong side when it came to key issues of his day. He opposed thinkers like Adam Smith and David Ricardo when it came to the Corn Laws, the tax on foreign imports that kept food prices high. Consequently, few scholars seem fascinated by the man Karl Marx called ‘a professional sycophant of the landed aristocracy’. As two new books make clear, though, there are good reasons to look beyond the population principle and reactionary politics when it comes to Malthus. He is among the most important thinkers of the modern era and we have still to get to grips with the nature of his effect on the world since 1798.

Robert J. Mayhew tackles these issues in a book that puts Malthus, a man who left behind few archival traces, at its heart. The first half of Malthus puts Malthus's life and work back into their late eighteenth- and early nineteenth-century contexts, helping us understand what motivated the second son of a ‘middling sort’ of radical father, who invited the French political philosopher Jean-Jacques Rousseau to stay when he visited Britain during the 1760s, to pursue the questions that would eventually lead to the population principle. The second half sees Mayhew explore Malthus's relationship with the world after the Essay. This involves thinking about both Malthus as a historical figure and ‘Malthusianism’, a label that identifies not only an interest in population trends but also various nineteenth- and twentieth-century campaign groups who used the term as a euphemism for contraception – a cause that Malthus himself never endorsed. The result is a fascinating transatlantic journey in which a reverend from Bath who studied at Cambridge and taught political economy to future employees of the East India Company became the inspiration, intended or otherwise, for ideas about the most pressing problems of every age since, including our own.

Mayhew's book teaches us many things. One point that comes through clearly is that we should view Malthus as something like a proto-social historian. We know that Malthus wrote his Essay in response to utopian Enlightenment social schemes, particularly William Godwin's, but we think little about how his critique was rooted in a description of the world experienced by most people. The population principle encapsulated the chasm between the privileged existences of Godwin and his fellow travellers and the lives led by the masses, more than one in four of whom would not live to see their fifth birthday. Another point is the often-overlooked breadth of Malthus's influence. As historians of economics know, John Maynard Keynes was fascinated by Malthus and took significant inspiration from his writings on economic demand. In this respect, Malthus's influence reaches right into the present and right into debates about how to respond to economic crises. Most scholars would identify the population principle as Malthus's greatest achievement but it was part of a broad oeuvre with far-reaching significance. Mayhew's achievement is to illuminate the connections between the interlocking parts of that oeuvre and explain its different influences with clarity and precision.

Alison Bashford also draws attention to these issues in a hugely impressive book that focuses on entangled concerns about population trends, food supplies, economics, and reproduction. Although Malthus is also her starting point, Bashford's concern is less with the fortunes of a particular theory or historical figure and more with understanding both the contexts in which those issues came to be entangled and how the intellectual contours of the debates about them have changed. Bashford's central preoccupation is the question of why there was never clear daylight between Malthus's two seemingly separate concerns – the population principle that fascinates historians of biology and his ideas about effective demand that interest historians of economics. Bashford recasts the concept of population, which Malthus helped shape, in new moulds. In so doing, she provides a framework – one that draws together economics, biology, ecology, imperialism and politics – that makes sense of a wide range of nineteenth- and twentieth-century debates and draws our attention to decisive but frequently undetected changes in them.

The outcome is an analysis of extraordinary geographic and intellectual reach. There has been much talk recently of global histories of science. Bashford's book is a remarkable contribution to that debate, not because it looks at more than one country, which is what many allegedly global histories boil down to, but because it illustrates how global thinking, encapsulated by the population concept, was central to the way scientists and politicians looked at the world. Since the early nineteenth century, issues like food supplies, national security, and welfare provision have been at the top of political agendas but their nationalistic overtones have often obscured their position in a global ecology. In the Cold War context, for instance, studying demographic trends in the developing world and identifying where there were large and therefore potentially disaffected young populations was a key part of identifying the next battleground between the US and Russia. This, in turn, was entangled with the history of plant genetics and the project to increase food supplies to those areas. This was geopolitics meets biopolitics.

Mayhew and Bashford's books are essential contributions to the literature on Malthus and the debates spawned by his enduring ideas about populations. Both books also end at more or less the same point: late twentieth-century debates about the ‘population bomb’ and their links to our own concerns about the impact we are having on our environment. This convergence is significant when it comes to considering the contributions that historians of science can make to debates beyond our field. The environment is one of the leading issues of our times but, as these books show, it is a concept with a long history involving thinkers and practitioners from a broad range of fields. Historians of the biological and the social sciences clearly have much to offer and the public debate would be enriched by hearing from them.