Families in developed nations who have more than two children should be subject to an overpopulation tax. I recently spoke with a father who had seven children because, as a Catholic, he believed the use of birth control methods such as withdrawal and contraceptives constitutes a mortal sin.Support for this belief has been drawn from passages in Genesis 38:8-10 where Onan was commanded to marry and have children with his dead brother’s wife. Verses 9-10 say: “And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother’s wife, that he spilled it on the ground … And the thing which he did displeased the LORD: wherefore he slew him also.”The Catholic Church advocates Natural Family Planning (NFP) — the avoidance of sexual relations when the female is fertile — as a biblically viable method of avoiding pregnancy.All other forms of birth control, excepting abstinence, are considered sinful, even within a marriage.To advocate NFP but to condemn methods such as withdrawal — “coitus interruptus” — is hypocritical. “Any action which either before, at the moment of, or after sexual intercourse, is specifically intended to prevent procreation  — whether as an end or as a means” is prohibited, wrote Pope Paul VI in the encyclical letter “Humanae Vitae.” But to engage in sexual intercourse only when the woman is unlikely to become pregnant is intended to prevent pregnancy to the same extent as Onan in Genesis, when he “spilled it on the ground.”The intent of the alleged sinner should determine the existence of sin, rather than the method he or she used.But the true problem is not whether certain people believe in the use of birth control. The fault lies with those willing to have children in excess of what is needed to maintain the population.The global population growth rate has decreased “from more than 2.0 percent to 1.5 percent a year over the past 30 years,” according to Worldbank.org. “But in the meantime, in absolute numbers it is still growing faster than ever before — by about 230,000 people a day.” The world population is projected to level off between 9 and 10 billion by the end of the century.Let’s look at this issue from a unique angle. “More children equal more carbon dioxide emissions,” writes New York Times reporter Andrew Revkin. “And recent research has resulted in renewed coverage of the notion that one of the cheapest ways to curb emissions in coming decades would be to provide access to birth control for tens of millions of women around the world who say they desire it.”Revkin asserts this is merely “a thought experiment.” This idea, although perhaps farfetched and certainly controversial, is a creative way to reduce the population and achieve a reduction in carbon emissions at a lower cost than using green technology.The decision to produce a large number of offspring is a personal decision which only the parents can make. But if they decide to have children in excess of the amount required to maintain the population, society should be compensated for the extra damage and burden placed upon it.In a time when global water and food shortages threaten the lives of millions around the world, the only responsible action is the use of birth control or the introduction of market incentives to curb the continued growth. God may want us to be fruitful and multiply, but I sincerely doubt he desires the destruction of the world through overpopulation.Nathan Shull is a 35-year-old finance junior from Seattle.  Follow him on Twitter @TDR_nshullContact Nathan Shull at [email protected]