The role of nurturing in determining one’s behavioral traits has been hotly contested. Historically, geneticists believed that behavioral traits are inherited. After all, many properties of the brain are genetically organized and don't depend on information coming in from the senses. Since active genes are essentially inherited, most traditional geneticists believe that nurturing environment plays little role in shaping one’s behavioral traits.
However, a new line of research indicated that methyl groups can activate dormant genes, bringing about a slew of changes much later in a person’s life. The methyl group works like a placeholder in a cookbook, attaching to the DNA within each cell to select only those recipes - er, genes - necessary for that particular cell’s proteins, telling the DNA what kind of cells to form. The first such observation was in which methyl groups activated by causes ranging from exposure to certain chemicals to changes in diet set off a cascade of cellular changes resulting in cancer. Because methyl groups are attached to the genes, residing beside but separate from the double-helix DNA code, their study is dubbed epigenetics - “epi” referring to Greek for outer or above.
Behavioral geneticists, encouraged by this discovery proved that traumatic experiences such as child neglect, drug abuse, or other severe stresses also set off epigenetic changes to the DNA inside the neurons of a person’s brain, permanently altering behavior. Similarly, through multivariate analysis, they proved that identical twins, in scenarios where one twin has gone through a life altering event, can have vastly different reaction to a stressful situation.
The primary purpose of the passage is to