During his State of the Union address, President Obama proclaimed that "every dollar we invest in high-quality early childhood education can save more than seven dollars later on, by boosting graduation rates, reducing teen pregnancy, even reducing violent crime."

Sounds impressive, but that "return on investment" figure reflects a lot of wishful thinking.

It appears to be based on the Perry Preschool Project, a study conducted more than 50 years ago in Ypsilanti, Mich. The Perry Project tracked 123 low-income, "at risk" children, 58 of whom were assigned to a treatment group that received high-intervention preschool services - everything from structured classroom instruction to weekly home visits.

Following the participants through age 40, the Perry program found those in the treatment group were more likely to be employed, to have graduated from high school, and to earn more than the control group. They were also less likely to have been arrested five or more times by age 40. As a result, Perry researchers claim a $7.16 return on every dollar invested.

But the findings from the Perry Preschool Project have never been replicated in state preschool programs. To make generalizations from this small, high-intervention program to a large-scale program would require what researcher Russ Whitehurst of the Brookings Institution calls "prodigious leaps of faith."

Indeed, it is more likely that the president's universal preschool proposal will produce results like those logged by state programs in Georgia and Oklahoma.

President Obama claimed that evaluations of these programs "show students grow up more likely to read and do math at grade level, graduate high school, hold a job, form more stable families of their own." But, in the words of the great song lyricist Ira Gershwin, "It ain't necessarily so."

Georgia has had universal preschool for all 4-year-olds since 1995, yet graduation rates have failed to significantly improve. In Oklahoma, home to taxpayer-funded preschool since 1998, graduation rates have actually declined.

And, as researchers Shikha Dalmia and Lisa Snell of the Reason Foundation point out, universal preschool has failed to reduce the reading achievement gap between white and black children.

In Oklahoma, the gap has remained completely unchanged - a 22 point gap on the National Assessment of Educational Progress.

Nor has it improved significantly in Georgia, where white children post reading scores 23 points - roughly two grade levels - higher than their black peers.

Not much of a return on hundreds of millions of dollars invested in universal preschool by Oklahoma and Georgia taxpayers.

Moreover, President Obama's universal preschool proposal might not even perform as well as the Georgia and Oklahoma programs. His far larger-scale program is far more likely to look like the failed Head Start program.

Despite taxpayer "investment" of nearly $8 billion per year, Head Start consistently fails to reap a return for either taxpayers or participating children.

In December 2012, the Department of Health and Human Services, the agency that administers Head Start, released a scientifically rigorous evaluation of more than 5,000 children participating in the program.

It found that Head Start had little to no impact on cognitive, social-emotional, health, or parenting practices of participants.

Policymakers guided by evidence should proceed with caution when considering large-scale preschool programs - especially since there is little evidence of unmet demand for them.

Approximately three-quarters of the nation's four-year-olds are already enrolled in some form of public or private preschool - with low-income families receiving subsidized services via state preschool programs and Head Start. President Obama's plan would subsidize middle-income and upper-income families - with no new benefit to low-income parents.

Excellence in early education requires abandoning the presumption that preschool for all is preferable to family care. It also requires eliminating ineffective programs, and reforming those that remain.

There are ways to improve early childhood education. Making Washington the nation's nanny is not the way to do so.

-Lindsey M. Burke is the Will Skillman Fellow in Education Policy at The Heritage Foundation.