“If you want to be happy for the rest of your life you need to make an ugly woman your wife,” or “if your rent is late and you might have to litigate, don’t worry, be happy,” are a few of the ways some popular singers verbalize ways to stay happy. The role that genes and environment play on happiness and the choices a person makes in life have been regularly investigated in studies involving criminals and twins.

If one was to begin exploring genes having an impact on personality at some point it would be necessary to analyze criminal behaviors. In her “Genetic and Environmental Influences on Criminal Behavior” paper Caitlin M. Jones of the Rochester Institute of Technology takes a closer look at genes and criminal behavior. “Having a genetic predisposition for criminal behavior does not determine the actions of an individual, but if they are exposed to the right environment, then their chances are greater for engaging in criminal or anti-social behavior,” she wrote.

Jones explores three general factors that help portray the role environmental factors have on personality. These factors that Jones says are commonly cited by researchers include studies on twins, family, and adoption.

In recent studies psychologists in Scotland conducted a study of almost 2,000 identical and non-identical twins. By using twins who had grown up together but a variation between some twins having the same DNA and others with a different make-up researchers were able to separate gene factors from environmental ones. Professionals at the Edinburgh University's Psychology Department then developed a survey about satisfaction and life for the twins.

The results showed a significant increase of identical twins having similar answers than the twins whose make-up is comparable to that of normal siblings. “We found that the same genetic influences which caused people to differ in terms of personality also caused them to differ in terms of their happiness. Things that siblings share in common from their environment and household don't dictate whether they are happy or not; it's their genes that do. Twins allow us to study this in detail,” said Dr Alexander Weiss who led the study.

The next area that Jones suggests looking at in the area of personality and genes has to do with adoption. In a 1995 article in the New Yorker by Judith Rich Harris, author of Pulitzer Prize nominated book “The Nurture Assumption” talks about a study called “The Colorado Adoption Project” where researchers at the University of Colorado recruited two hundred and forty-five pregnant women from the Denver area who planned to give up their children for adoption.

In short, results of tests done at regular intervals throughout the upbringing of each child showed them to be “no more similar in personality or intellectual skills to the people who reared them, fed them, clothed them, read to them, taught them, and loved them all their lives than they were to any two adults taken at random off the street,” wrote Harris.

When it comes to family, certain genes can affect how humans react to specific factors having to do with upbringing. The gene monoamine oxidase A, MAOA, which has been in the news lately, greatly affects behaviors like aggression. The function of the gene serves to break down brain chemicals that are shown to be involved in several aspects of behavior.

Research performed at King's College in London examined boys who had been mistreated while growing up. Researchers found that those who produced low levels of MAOA were much more likely to indulge in defiant conduct.

Humans who have low levels of the gene are not necessarily predisposed toward violent tendencies. However, when those people are raised in an environment that is less than nurturing it becomes affected.

“The results provide evidence that a person's genetic make-up can influence their sensitivity to environmental factors.” King’s College researcher and Professor Terrie Moffitt encapsulated. “These findings may also partly explain why not all victims of maltreatment grow up to victimize others - some genes may actually promote resistance to stress and trauma.”