All of us are in denial. Without it we couldn't get through life. Were we to confront the implications of mortality, were we to comprehend all we have done to the world and its people, we wouldn't get out of bed. To engage comprehensively with reality is to succumb to despair. Without denial there is no hope.

But some people make a doctrine of it. American conservatism could be described as a movement of denialogues, people whose ideology is based on disavowing physical realities. This applies to their views on evolution, climate change, foreign affairs and fiscal policy. The Vietnam war would have been won, were it not for the pinko chickens at home. Saddam Hussein was in league with al-Qaida. Everyone has an equal chance of becoming CEO. Universal healthcare is a communist plot. Segregation wasn't that bad. As one of George Bush's aides said: "We're an empire now, and when we act, we create our own reality."

Collective denial has consequences. A new study by the US Centres for Disease Control (CDC) shows that during the latter years of the Bush presidency, America's steady progress in reducing teenage pregnancies and sexually transmitted diseases was shoved into reverse.

Between 1990 and 2004, the birth rate among teenage girls fell sharply: by 46% for 15- to 17-year-olds. The decline was unbroken throughout these years. (The same thing happened in the rest of the western world, though about 20 years earlier). But between 2005 and 2006, something odd happened: the teen birth rate increased by 3%. In 2007 it rose by another 1%. I think most people would agree that this is a tragedy. According to the UN agency Unicef, women who are born poor are twice as likely to stay that way if they have children as teenagers. They are more likely to remain unemployed, to suffer from depression and to become alcoholics or drug addicts (all references are on my website). Similarly, the incidence of gonorrhoea dropped for more than 20 years, then started to rise in 2004. After a long period of decline, syphilis among teenage boys began to increase in 2002; among girls in 2004.

The CDC makes no attempt to explain these findings, but the report contains four possible clues. The first is that between 1991 and 2007, the percentage of high school students who had ever had sex declined. So did the number of their sexual partners, and their level of sexual activity. But from 2005 onwards there was a levelling or reversal of all these trends. The second possible clue is that while the use of condoms among high school students rose steadily from 1991 to 2003, it stagnated then declined between 2003 and 2007. Towards the end of the Bush years, schoolchildren began abandoning condoms at the same time as their sexual activity rose.

The third clue is provided by the shocking data from the Hispanic community. Adolescent Hispanic girls have less sex than their non-Hispanic classmates; but they have three times as many children as non-Hispanic white teenagers. Why? Because they are less likely to use contraceptives, probably because of the doctrines of the Catholic church.

But perhaps the most interesting clue is this one. The CDC has published a map of trends in the teenage birth rate. I ran it against a political map of the Union and found this: nine of the 10 states with the highest increase in teenage births voted Republican in the 2000 and 2004 presidential elections. (Eight of them voted for McCain in 2008.) Among them are the Christian conservative heartlands of Kentucky, Alabama, Mississippi, Louisiana and Oklahoma. These are the places in which Bush's abstinence campaigns were most enthusiastically promoted.

Bush did not invent sex education without the sex. Clinton's last budget set aside $80m for abstinence teaching. But by 2005 Bush had raised this to $170m, and engineered a new standard of mendacity and manipulation. A congressional report in 2004 explained that programmes receiving this money were "not allowed to teach their participants any methods to reduce the risk of pregnancy other than abstaining until marriage. They are allowed to mention contraceptives only to describe their failure rates." The report found that over 80% of the teaching materials "contain false, misleading, or distorted information about reproductive health". They suggested, for example, that condoms do nothing to prevent the spread of STDs, that 41% of sexually active girls and 50% of homosexual boys are infected by HIV, and – marvellously – that touching another person's genitals "can result in pregnancy".

While "abstinence-plus" campaigns (teaching contraception while advising against sex) are effective, a long series of scientific papers shows that abstinence-only schooling is worse than useless. A paper published in the British Medical Journal found that abstinence programmes "were associated with an increase in the number of pregnancies among partners of young male participants". An article in the Journal of Adolescent Health found that although teenagers who have taken a pledge of abstinence are less likely to have sex before marriage, and then have fewer sexual partners, they have the same overall rate of infection as the kids who haven't promised anything. This is because the pledgers are less likely to use condoms, take advice or go to the clinic when they pick something up. Most teenagers (88%) who have taken the pledge end up breaking it. But, like the campaigners, they are in denial: they deny that they are having sex, then deny that they have caught the pox.

A study published by the American Journal of Public Health found that 86% of the decline in adolescent pregnancies in the US between 1991 and 2003 was caused by better use of contraceptives. Reduced sexual activity caused the remainder, but this "ironically … appears to have preceded recent intensive efforts on the part of the US government to promote abstinence-only policies". Since those intensive efforts began, sexual activity has increased.

Unicef, when it compared teenage pregnancy rates in different parts of the world, found that the Netherlands had the rich world's lowest incidence – of five births per 1,000 girls – and the US had the highest: 53 per 1,000. Unicef explained that the Dutch had "more open attitudes towards sex and sex education, including contraception". There was no "shame or embarrassment" about asking for help. In the US, however, "contraceptive advice and services may be formally available, but in a 'closed' atmosphere of embarrassment and secrecy".

Barack Obama's new budget aims to change all this, by investing in "evidence-based" education programmes. The conservatives have gone ballistic: evidence is the enemy. They still insist that American children should be deprived of sex education, lied to about contraception and maintained in a state of medieval ignorance. If their own children end up with syphilis or unwanted babies, that, it seems, is a price they will pay for preserving their beliefs. The denialogues are now loudly insisting that STDs and pregnancies have risen because Bush's programme didn't go far enough. The further it went, the worse these problems got.