"Teenagers are very good liars," said Hirsch. "They know what they want and we as parents want to trust them. But our research shows that lying starts as young as age 3. In fact, as soon as we start to know the difference between lies and the truth, we start to lie."
Hirsch's latest book, "How to Tell If Your Teenager Is Lying and What to Do about It" (Hilton, $17.95), teaches the reader ways to detect lies, and includes tips on how to establish communication to prevent deceit.
"The most common lies happen when something is at stake, such as a reward or a punishment," he said. Hirsch also discovered that while many people lie about their age, women are more likely to lie about their weight and height.
"The tricky part is explaining to your child that not all lying generates a negative result," said Hirsch. "You may have a bride who isn't the most beautiful woman, but you're surely not going to tell a bride on her wedding day that she's ugly. And sometimes your job may require you lie, like if you're protecting your country. The difference is whether you are lying to help yourself and gain something for personal benefit from a narcissistic perspective, or if you are doing so to be empathetic and help others."
Hirsch said anyone can be taught to pass a lie detector, and it's important to remember that stress and lying can cause a lot of the same physical symptoms.
"Be sure to notice how people act during truth periods so you can notice the difference if they are showing some of the lying behavior patterns," he said. "And the tips I give won't help if you're dealing with a sociopath. This is someone with no moral compass, so they won't have the same physical reaction as the person who knows their behavior is deceitful."
Here are some things Hirsch said to look for if you wonder whether your teenager is lying to you.
The itchy nose: "When you lie you feel guilty about it, and this causes an increase in blood pressure and heart rate," said Hirsch. "There is an engorgement of tissue in the nose which causes it to itch. We've named this the Pinocchio effect."
Speech errors: "Be wary if someone stutters out of nowhere or says 'uh' and 'er' multiple times," he said. "Also, saying 'would not' instead of 'wouldn't' or 'could not' instead of 'couldn't'. When you lie, you spell it out. President Clinton did this during the Lewinsky scandal for example."
Leaning in: "Liars will lean in when they're telling the lie because they want to encompass you into the lie," Hirsch said.
Drinking or swallowing: "A liar will drink more often or swallow more than someone telling the truth," he said.
Licking and tightening the lips: "The liar uses the tongue to lick the lips and tighten the mouth, as if to prevent anything from getting out," he said.
Crossing your arms: "This creates a barrier, almost separating the liar from the one being lied to."
Fidgeting: "This can be with a pen, papers or eyeglasses. The liars have a hard time sitting still, and this is because of the adrenaline that is pumping through their system—which is a direct physical reaction to telling a lie."