"If it looks like a duck, walks like a duck, and quacks like a duck, it's probably a duck."
- Senator Joseph McCarthy,
in a 1952 speech, suggesting a method for identifying communists and communist sympathizers.
We lawyers like to use the above phrase. I have used it myself in court.
Joseph McCarthy, quoted above, himself was a lawyer and a judge before serving in World War 2 as a marine. On completion of his war service he became a politician and hunter of communists, both real and imagined. He was not, however, the originator of the saying or the first to use it.
The phrase is often referred to as the “duck test”” and is a crude, but often effective, form of reasoning that eliminates alternative, unlikely possibilities and identifies a most likely or most correct assessment.
The saying is sometimes confused with the following:
1. Occam’s Razor: “When there are two competing explanations for an event, the simpler is the more likely.”
2. Skeptic’s Test: “Where there are two competing explanations for an event, one explanation is consistent with laws of science, reason and logic and the other requires suspension of belief in such laws and principles, then the former should prevail until the latter is proven.
Although there is some overlap, they are separate and distinct laws and principles.
Assume a child awakes on Christmas morning to find presents at the foot of the bed. There are two explanations possible: Mum and Dad placed them there when little Johnny was asleep, or Santa delivered them. According to the skeptics test, it should be accepted that Mum and Dad delivered them until it is proven that Santa did it.
Occam’s Razor would also support that explanation as the simpler of the two.
Now assume that Johnny awakes to find the presents at the foot of his bed. He later finds identical wrapping paper and ribbon in the hall cupboard, plus he noticed Mum coming home from the shops with similar sized parcels. Again there are two possible explanations. One is that Santa brought the presents and it is entirely coincidental that Mum and Dad have the same wrapping paper and that he saw Mum coming home with similar parcels. The second is that that Santa left them. On the duck test, Johnny should assume Mum and Dad were the givers of the presents, not Santa.
The more notable uses of the phrase, thereby popularising it, have been in connection with persons suspected of communism:
US poet Whitcomb Riley (1849–1916) is believed to have originally coined the phrase about one hundred years ago in writing:
“when I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.”
Emil Mazey (1913-1983), the secretary-treasurer of the United Automobile Workers for 33 years, said at a labour meeting in 1946 when he accused someone of being a communist:
“I can’t prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks—I’m certainly going to assume that he IS a duck.”
In 1950 Richard Patterson Jnr, the US ambassador to Guatemala accused the Guatemalan government of being communist and stated:
“Suppose you see a bird walking around in a farm yard. This bird has no label that says 'duck'. But the bird certainly looks like a duck. Also, he goes to the pond and you notice that he swims like a duck. Then he opens his beak and quacks like a duck. Well, by this time you have probably reached the conclusion that the bird is a duck, whether he's wearing a label or not."
Douglas Adams’ novel Dirk Gently’s Holistic Detective Agency uses a variant:
“If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family anatidae on our hands.”
(Click on the image to enlarge)