What makes us who we are? The biology of genetics or the environmental tinkering of parents? A highly controversial study carried out from the late 1960s until 1980 sought to settle that contentious score by zeroing in on a population that could yield the richest results: twins. A child psychologist and child psychiatrist teamed up with a New York adoption agency to split up a group of 13 twins and triplets and track their life courses as they grew up in vastly different households and without the knowledge of being a multiple [source: Richman]. The researchers wanted to test over the life-term whether nature or nurture would steer the pairs and trios sharing identical genetic information toward similar or disparate paths.
In 2004, a pair of twin sisters unwittingly involved in the study met up for the first time. Despite being raised in vastly different families, it turned out that Paula Bernstein and Elyse Schein both ended up serving as editors of their high school newspapers and studying film at liberal arts colleges [source: Richman]. Even down to their political affiliations, the twins were clearly cut from the same cloth [source: France].
Psychology researcher Judith Harris might point to that anecdote as evidence in favor of her nature-over-nurture hypothesis, which ignited major backlash within the psychological community in 1998. As detailed in her book "The Nurture Assumption," Harris contends that parents don't really make much difference in how their kids turn out [source: Begley]. The parents-may-wreck-all logic was instigated, Harris argues, by Freud and his fretful theory of psychosexual development, which reimagined little boys and girls as Oedipuses- and Electras-in-waiting who might develop into maladjusted adults if their parents don't pamper or potty train them properly.
Harris' thesis contends, on the contrary, that heredity and peer relations –- i.e. the extra-familial environment -- are actually pulling the puppet strings of destiny [source: Harris]. Furthermore, more recent behavioral genetics studies calculate that DNA wields up to 50 percent of the power when it comes to molding how kids conduct themselves, which can have an ocean's-worth of ripple effects down the road [source: Azar].
Behavioral psychologists couldn't disagree more. In some academic circles, the prevailing belief is that parenting during the formative years of infancy and childhood imprints patterns and behaviors so lastingly, it can be referred to as "programming" [source: Gross]. Intervention studies on parenting practices, for instance, have found that positive adjustments to how adults interact with and discipline their young kids can improve the children's social skills, academic performance and focus [source: Azar]. As go moms and dads, these mental health experts maintain, so go sons and daughters.
The intellectual boxing match between shrinks and geneticists likely won't end anytime soon, because the various and sundry ingredients of child development are -- so far, at least -- impossible to measure out precisely. Studies can draw compelling correlations between, say, how often mom or dad reads to their preschooler and the child's language arts skills, or particular gene sequences and impulse control. But researchers also have acknowledged that quantifying the long-term impacts of these kinds of factors in adulthood is beyond their grasp.
That said, those correlative relationships aren't worthless. For public health advocates in particular, parents' sway on kids' habits has possibly life-threatening implications.