If a society is judged by how it treats its most vulnerable members, the United States just received an incredibly unflattering judgment.

A new study published by the United Nations Children's Emergency Fund, or UNICEF, ranked the wealthiest countries of the world by the well-being of their most disadvantaged children. Out of 41 countries, the U.S. ranked No. 18 overall.

For context, the U.S. ranks No. 1 in total wealth.

The study took a comprehensive approach, comparing the gap between children at the very bottom to those in the middle across a range of criteria – including household income, educational achievement and self-reported health and life satisfaction. The central question was this: How far do countries let those at the very bottom fall?

In the United States, the answer seems to be distressingly far.

On income inequality, the U.S. ranked No. 30, coming in behind countries like Turkey, Estonia and Slovakia. Driving the gap is the fact that one in five American children lives in poverty – a figure among the worst in the world. On education, the U.S. fared better at No. 10, with a relatively small achievement gap between those at the bottom and those at the middle, but plunged to No. 21 on life satisfaction – coming in well behind even recession-plagued countries like Greece, Spain and Portugal.

It shouldn't be surprising that Scandinavian countries dominated the overall rankings, with Denmark at the top and Finland, Norway and Switzerland all tied for second. These countries have made massive investments in a social safety net that ensures those at the bottom have opportunities to get ahead. Those same investments in the U.S. have stagnated.

Buried deep in the voluminous report is this sentence: "Growing up in unequal, harsh social environments may pose a barrier for children to a healthy, happy, and productive life."

Indeed. The gap created by an unequal start in life begins very early on and can have a lasting impact on a child's life. According to the report, "From as early as the age of 3, children from more affluent backgrounds tend to do better in cognitive tests." Further, at age 5, children from poor families are three times more likely to be in the bottom 10 percent in cognitive ability than nonpoor children.

Investments in programs like guaranteed universal early childhood education, or pre-K, could improve prospects for children at the bottom. Currently the United States ranks No. 26 in preschool participation and No. 21 in total investment in early childhood education relative to country wealth.

Universal pre-K is not a radical idea. President Obama proposed such a plan in his 2013 State of the Union address. And earlier this year, I coauthored a plan for how it could be funded by closing the carried interest loophole, a tax break for hedge fund managers so egregious that even Donald Trump says he supports closing it.

Unfortunately, the prospects for passing that plan in the near term look grim. On a further sour note, even universal pre-K wouldn't close the gap between kids from affluent families and those from poorer families.

The real reason for the underachievement of those at the very bottom is remarkably simple: They're at the very bottom. Their families lack income and wealth and the basic necessities that these afford. Raising the minimum wage and instituting a guaranteed basic income could ameliorate these ills, but these too are a long way from becoming federal law.