… by which I mean Person A slipping something into Person B’s food or drink; accidents, industrial problems, and medical murders are outside the scope of this essay.
Classically, poisoning was seen as a woman’s weapon; manly men just stabbed you or whapped you over the head or strangled you, as one does. Poisoning was sneaky and evil because you didn’t, however briefly, see it coming. For 17 years in the 16th century, poisoning was prosecuted as high treason and earned you death by boiling. (Presumably without arsenic or other additives.) Anybody who reads Golden Age murders will be intimately familiar with the varying symptoms of arsenic, strychnine, digitalis, and various substances unknown to science – the last being also outside our scope.
Nowadays you’re much more likely to be shot (in the U.S.) or blunt/pointy/flammable objected (elsewhere) than poisoned. This makes murder mystery authors sad. What happened?
Improved availability of contraception
Physical mobility of the population
Regulation of poisonous substances
The birth of forensic medicine
For much of English and American history, most of the population were stuck wherever they were brought up. In particular, many people lived in multi-family houses, with the resource-controlling ancestors, descendants, and a passel of babies all stuffed together under one roof. If you couldn’t stand your mother-in-law, too bad; she was going to be sneering at the dinner table until she passed on, and frankly she was healthier than you were. If your husband was beating you or just driving you up a tree, too bad; “till death do us part” was pretty much the rule, unless you were male, very very wealthy, and able to buy a divorce. If you kept having babies you couldn’t feed, abortion was haphazard and depended on knowing the right people/plants.
All of these problems were most easily solved by murder. You couldn’t move away; you couldn’t divorce and remarry; you couldn’t stop the babies coming. (For babies, there was the bonus that you could insure them and collect the money.) You were stuck with your family. If female, you were at least supervising and very likely cooking the meals. The solution was obvious. The solution was made even more obvious when you consider that most households contained lethal substances for killing flies, rats, and weeds. You would naturally keep arsenic around to solve these problems, as well as for cosmetic uses. You got arsenic – or strychnine, or prussic acid, or whatnot – by strolling up to the apothecary and requesting it. If questioned, you’d just explain that you wanted to get rid of rats; you would prudently not append “Like the one I married”.
Irritant poisons, like arsenic, do nasty things to the digestive system, causing noxious substances to issue from both ends. However, in a pre-sanitation age, people died of gastrointestinal ailments all the time. If no doctor is called, or if the doctor isn’t suspicious, there’s no reason to think that the guy who just died was poisoned. (Strychnine being a quite spectacular exception.) Even if the servants or the doctor are suspicious, the only way to prove that the food was poisonous is – if somebody saved the food or the fluids – to feed them to an unfortunate animal, usually a dog.
So, motive, means, and opportunity. The authorities tried to give the poisonee (or his/her estate) a sporting chance by regulating the sale of certain poisons. Arsenic had to be colored so that it was easily distinguishable from sugar or salt or whatever; people who bought certain poisons had to register at the place of purchase. The major change, however, was the invention of forensic testing; Wikipedia has a nice summary of the progress in detecting arsenic. (Readers of Dorothy Sayers’s Strong Poison will be familiar with the Marsh test.) Detecting arsenic and other inorganic chemical compounds happened fairly early; detection of strychnine, digitalis, and so on required substantial advances in chemistry. As sanitation moved on, there were far fewer “gastroenteritis” deaths, and so poisonings were less-well camouflaged. By the mid-19th century, at least in urban areas, poisoning somebody had a higher, if by no means absolute, risk of sending you to the gallows.
And now we come to mobility, divorce, and contraception. Trains made it far easier for the middle classes to move from one city to another; labor migration to the cities made it easier to move from one house/apartment to another. Again for the middle classes, the gradual loosening of divorce laws made it much more possible to get away from your horrible spouse. And contraception made it possible to prevent a crowded house before rather than after the birth. Banning the insurance of infants removed a financial incentive. Put all these together, and it’s much harder to write a cozy poisoning in 2015 than in 1925.
I highly, highly recommend Katherine D. Watson’s Poisoned Lives: English Poisoners and Their Victims and Deborah Blum’s The Poisoner’s Handbook: Murder and the Birth of Forensic Medicine in Jazz Age New York if you want to know more about the sociology of poison in the United Kingdom and the USA.