Generic sentences express generalizations about kinds, such as "tigers are striped", "ducks lay eggs", and "ticks carry Lyme disease". I present and review emerging evidence from adults and children that suggests that generics articulate cognitively default generalizations — i.e., they express basic, early-developing inductive generalizations concerning kinds. Further evidence suggests that these generalizations don't depend solely on information about prevalence. For example, "ticks carry Lyme disease" is accepted, but "books are paperbacks" is not, despite the fact – well-known and acknowledged by participants – that paperbacks are much more prevalent among books than Lyme-disease-carrying is among ticks. Similarly, both adults and preschoolers understand that, e.g., only female ducks lay eggs, yet they are more likely to accept "ducks lay eggs" than "ducks are female". Rather than depending solely on information about prevalence, these primitive generic generalizations are sensitive to a number of content-based factors, such as whether the property in question is dangerous or otherwise striking (as in "ticks carry Lyme disease"), or is an essential or characteristic property of the kind (as in "ducks lay eggs"). This suggests that our most basic means of forming inductive generalizations about kinds is not guided by prevalence alone, but also reflects our nature as learners.
When Prof. Leslie says that "these primitive generic generalizations are sensitive to a number of content-based factors", she's relying on experimentation as well as introspection, as documented in several of her recent publications, such as Leslie, Khemlani, & Glucksberg, "Do all ducks lay eggs? The generic overgeneralization effect", Journal of Memory and Language, July 2011, or Khemlani, Leslie, & Glucksberg. “Inferences about Members of Kinds: The Generics Hypothesis". Language and Cognitive Processes, forthcoming.
As she points out, the relationship between mere prevalence and such generic statements is a very weak one. Everyone knows that "Ticks carry Lyme Disease", although only a minority of ticks do so (14% in one study). Everyone knows that "Mosquitoes carry West Nile Virus", though the highest infection rate found in the epicenter of a recent epidemic was estimated at 3.55 per thousand (and the rate was essentially zero outside of the epicenter).
And, of course, everyone knows that "Ducks lay eggs" and "Lions have manes", though in each case the prevalence is at most 50%. In contrast, people generally reject the generic statement that "Books are paperbacks", although it's true that the majority of books are in fact paperbacks.
She suggests that among the "content-based factors" involved in such judgments are (1) whether the property is perceived to be characteristic of the kind, or an essential property of the kind; (2) whether the property is striking and/or dangerous; and (3) whether the members of the kind who lack the property have an equally salient alternative property.
This analysis helps explain why generic statements work in ways that once led me to propose banning them from scientific discourse. More precisely, I half-seriously recommended that a certain type of generic comparison should be avoided ("Mandatory treatment for generic plurals", 9/13/2009):
I propose a voluntary ban on the use of generic plurals to express statistical differences, especially in talking to the general public about scientific results in areas with public policy implications.
In other words, when we're looking at some property P of two groups X and Y, and a study shows that the distribution of P in X is different from the distribution of P in Y to an extent that is unlikely to be entirely the result of chance, we should avoid explaining this to the general public by saying "X's have more P than Y's", or "X's and Y's differ in P", or any other form of expression that uses generic plurals to describe a generic difference.
This would lead us to avoid statements like "men are happier than women", or "boys don't respond to sounds as rapidly as do girls", or "Asians have a more collectivist mentality than Europeans do" — or "the brains of violent criminals are physically and functionally different from the rest of us". At least, we should avoid this way of talking about the results of scientific investigations.
The reason? Most members of the general public don't understand statistical-distribution talk, and instead tend to interpret such statements as expressing general (and essential) properties of the groups involved. This is especially true when the statements express the conclusions of an apparently authoritative scientific study, rather than merely someone's personal opinion, which is easy to discount.
This proposal focused on specific types of generic statements, namely those that describe differences in group distributions in terms of a comparison of generic plurals. Looking over some of the many posts in which I've complained about the misuse of generic plurals, both by journalists and by scientists, most of them involve statements that make such explicit comparisons of group distributions. Thus
Having a sister makes you happier. ("How powerful is sisterhood?", 10/27/2010).
Women apologize more than men do. ("Offenses and apologies", 10/10/2010).
Conservatives are more squeamish than liberals. ("Physiological politics", 2/15/2010; "Icktheology", 2/18/2010)
Men are happier than women. ("The Happiness Gap is back is back is back is back", 9/30/2009)
The male retina is substantially thicker than the female retina. ("Retinal sex and sexual rhetoric", 5/20/2008)
Women are more talkative than men. ("Gabby guys: the effect size", 9/23/2006)
Girls hear better than boys do. ("Leonard Sax on hearing", 8/22/2006.
The others generally involve implicit comparisons, e.g.
Heavy multimedia users have trouble filtering out irrelevant information. ("Are 'heavy media multitaskers' really heavy media multitaskers?", 9/4/2010)
Babies cry in their native language. ("Native wails", 11/6/2009
In all of these (explicit or implicit comparative) cases, I've argued that the results are presented in a way that misleads the public — and in some cases, the use of generic plurals in comparatives seems to mislead the scientists themselves.