It is as close to conventional wisdom in education as it gets – if parents want their children to succeed in school they need to introduce them to learning as early as possible both at home and by enrolling them in a preschool. For parents who live in urban areas like New York City, that means planning for their child's education before they are even born and beginning an arduous application process a full year before the first school bell.
Now, a number of recent studies have called the usefulness of preschool into question. Some have suggested that if parents are the type who agonize over which preschool will work best for their child are also likely to be the kind of parents whose kids won't lose much by skipping it altogether. In other words, parents who take their children's education very seriously already devote so many resources to academic enrichment that the addition of a teacher, a few classmates and some colorful toys aren't likely to make a big difference.
It's hard to tease out the effects of preschool on a child. Part of the problem is self-selection: Compared with kids who skip preschool, kids who attend usually have more well-to-do, encouraging parents who read and do puzzles with them at home.
Children who don't go to preschool are usually from more disadvantaged families, which means they watch lots of TV and are yelled at more than they are praised, which some researchers believe can stunt cognitive development.
Children in well-to-do families get a non-trivial amount of exposure to things that a preschool focuses on teaching. For example, a recently published study which followed a number of families for several years found that kids of professional parents hear more than 30 million words in the first three years of their lives. In contrast, children from low-income families are only exposed to one third as many over the same period.
The home environment, not the quality of the preschool program, is what has the most impact on childhood development, researchers found. Indeed, the children most in need of this kind of early childhood intervention are the ones who are less likely to receive it – those who are born to low-income parents. And the main advantage they gain is that being in school puts them in an environmental that their richer peers already get just from hanging around their folks.
In other words, a bad home situation becomes a much smaller problem when your kid goes to preschool; when you have a good home environment, preschool doesn't really matter. (Granted, children from poor families tend to go to lower quality preschools than wealthy kids do, but for them, a bad preschool is usually better than nothing.)