The 2004 General Social Survey shows that, if two adults in 2004 were the same in age, sex, income, marital status, education, race, religion, and politics--but one had kids but the other did not--the parent would be about seven percentage points less likely than the non-parent to report being "very happy."
Furthermore, the average happiness of adults--correcting for all the other personal characteristics like before--falls as more kids are added to the family. This is true, however, only up to four kids--with the fifth, parental happiness appears to start to rise again. The reason for this is because, in modern America where the average number of children per family is close to two, a large brood generally belongs to a certain unusual type of heroic parent (who fall into the category of saints and martyrs).
These facts should not be interpreted as evidence that non-parents are, as a group, happier people than parents. People who have the most kids today generally have other traits that more than offset the children in their happiness. For example, political conservatives have far more children than liberals (41 percent more kids per couple in 2004), but their worldview brings them up more than their kids bring them down: 42 percent of conservative parents are very happy, versus 21 percent of liberal non-parents. (In case you're wondering, 52 percent of conservative non-parents are very happy.)
I am suspicious of these happiness comparisons between parents and non-parents. It seems to me that moving to parenthood changes a person's preferences so dramatically that it hardly seems fair to use the same measuring rod for happiness. Parents and non-parents derive happiness from very different things. Children draw us out of our self-centeredness in a way that nothing else can do. Taking happiness in another peron's happiness or success surely counts as a different sort of thing than finding happiness in the enjoyment of ordinary consumption items.
Or, you could look at it from the reverse side: parents have different worries and concerns than non-parents. There are a whole host of problems than simply aren't on the radar screen of the non-parent.
And it is simply not the case that children are a net burden on society. Brooks points out the social benefits to government revenues.
Economists estimate that the net benefits to society from children are, on average, significant and positive. Balancing the negative and positive socioeconomic impacts from children, one well-regarded study from 1990 in the Annals of the American Academy of Political and Social Science placed the benefit in net government revenues in excess of $100,000 per American child--a number that has obviously greatly increased since that time.
What we choose to do with this public surplus per child is obviously up to us--it reflects our society's values. As our population rises, we can use our resulting public wealth increases to ensure the preservation of our natural environment, for example. To argue against human reproduction to save the planet amounts to arguing that lowering our prosperity is the best strategy to cut resource consumption and greenhouse gases. A common complaint about the current environmental movement is that it cares more about trees than people. This complaint is certainly not weakened by arguments for negative population growth.
More children also have the potential for more productivity in the private sector as well as in generating tax revenue for the government.
Read all of Brooks' article here.
More on the connection between parenthood and happiness here.