Most of us know the story of Achilles, the Greek warrior whose mother tried to protect him from an early death by submerging him in the magical waters of the river Styx. Despite her best efforts, she missed dunking one heel that she gripped as she dipped him upside down in the river. And as fate would have it, a poison arrow to this single weak spot led to Achilles' demise. By most accounts, he was still a very young man.

The myth reflects the way we tend to think about vulnerability—as a fixed liability almost destined to play out badly in battle or under other kinds of stress. But imagine if we could pen an alternative ending to this tragedy. Imagine that Achilles never went to war and that his famous heel ultimately became a source of strength. Consider, for example, a version of the story in which his exposed heel made him a superfast runner in the warm glow of the sun. Such a rewrite reflects what we are beginning to learn about how so-called environmentally sensitive children—considered fragile in the face of many kinds of adversity—can excel under more favorable circumstances.

The fact is that most kids, even some of the least fortunate in society, show a remarkable psychological resilience to life's misfortunes. Swedish folklore describes them as “dandelion children,” able to put down roots and survive in the rockiest soil. But other little people—the environmentally sensitive ones—are especially vulnerable to the ill effects of hardship. Countless studies reveal that when these kids grow up poor, are in troubled families, or are mistreated, discriminated against or neglected, they run a far greater risk of developing a host of mental health and behavioral problems, compared with the rest of the population. For this reason, many scholars refer to them as “orchid children,” prone to wither in harsh conditions.

In recent years evidence has been mounting in support of something rather unexpected: what makes these orchid kids so susceptible to negative environmental influences—as far as we know, a mix of different behavioral and biological traits—also renders them the most likely to benefit from extra support and nurturing. With a little greenhouse care, they thrive—so much so that they even outperform their less sensitive peers. Meanwhile the attributes that foster resilience in the dandelions, such that they do not readily succumb to setbacks, also appear to make them less responsive to various kinds of enrichment.

This differential susceptibility, as I refer to it, to both good and bad environments raises tough questions for parents, policy makers, teachers and concerned citizens alike. Should we seek to identify the most impressionable children and disproportionately target them when it comes to investing our attention and scarce intervention and service dollars? Is this the best approach to promote well-being and prevent future difficulties? I believe the answer may be yes. First, we need to be able to distinguish these rare blooms from the more resilient majority. And that will not be easy. No one characteristic defines them, but we do know that many seem to start off in life as difficult babies, and increasingly we are able to identify them using a range of genetic markers.

For better and for worse

Some children are just more demanding to raise: as infants, they fuss a lot, have trouble sleeping and are easily upset by new situations. Decades of research have demonstrated that if these irritable infants face early struggles—perhaps in the form of harsh discipline or insensitive parenting—they are more likely than other babies to become aggressive, depressed, anxious or disobedient as older children and teens. The less well-known flip side of the coin is this: when such challenging kids are reared in supportive or even neutral conditions, they can blossom both socially and emotionally. They thus wind up at the very top or bottom of the pile depending on the overall tenor of experiences in their formative years.

This “for better and for worse” pattern is emerging in a growing number of investigations. In 2009 Michael Pluess, then my graduate student and now at Queen Mary, University of London, and I first discovered it when we analyzed data from the Study of Early Child Care and Youth Development, initiated by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). Between 1991 and 2007 this large research project followed some 1,300 children in 10 U.S. cities from birth to age 15. In our evaluation, we focused on 968 of these kids, about whom the researchers had collected a wealth of data from birth to age five. When the children were babies, they tested their temperaments. When they were toddlers, they recorded how sensitive and stimulating their care was at home, at day care and in preschool. And later on, when these kids started elementary school, they asked their teachers to appraise their behavior.

Our results revealed something interesting. Contrary to what many people might expect, the five-year-olds who had received the best early care did not always wind up among the best behaved, according to their teachers, who knew nothing about the children's backgrounds. Instead, just as Pluess and I had anticipated, only the temperamental infants, when well cared for, reliably matured into model kindergarteners. When these more irritable babies received poor care, they typically became the most aggressive and disobedient in class.

A few years later psychologist Jude Cassidy of the University of Maryland and her colleagues found something similar. They conducted a more rigorous experimental test of the effects of parenting on newborns who were easily distressed. This team randomly assigned 220 poor mothers to either the Circle of Security intervention program or a control group. The intervention taught skilled parenting techniques to promote secure infant-mother attachments, whereas sessions for the control group focused on common concerns for new parents, such as sleeping and feeding. Mothers participating in the intervention did indeed become more adept caregivers. But their more attentive ways only resulted in more secure children if, before the training, the researchers had rated their babies as being particularly irritable. For the less fussy lot, parenting ability held far less sway.

Why might sensitive, responsive care make such a large difference in the development of temperamental children—and have less of an impact on everyone else? I believe that the hallmark tendencies of difficult kids—to be negative emotionally, inflexible and sometimes more active—all suggest that they possess especially delicate nervous systems on which all experiences, both good and bad, register with real force. Some of this sensitivity may develop via the body's stress response. For example, research shows that when a pregnant woman suffers extreme stress—perhaps as a result of actual or threatened violence—it forecasts greater negativity and physiological reactivity in her infant. These babies cry more and are more readily startled. Flooded with higher levels of the stress hormone cortisol in the womb, they emerge primed to mount stronger fight-or-flight responses to any kind of crisis.

But there are other mechanisms at work, which may or may not have anything to do with how the body handles pressure. Some kids may pay closer attention to their surroundings, analyzing information at a deeper level and thus being more affected by it. Some may have more reactive amygdalae, brain structures responsible for processing emotion; they could therefore experience feelings more acutely. Others might be more sensitive to punishment, thanks to underlying differences in their serotonin neurotransmitter system, which is involved in regulating mood. And some kids might also, or instead, react more intensely to rewards because of variations in how their dopamine neurotransmitter system works.

Vulnerability vs. possibility

Many of these differences reflect a child's DNA, and in fact researchers have tied environmental sensitivity to several genes, including the serotonin transporter gene 5-HTTLPR, a dopamine receptor gene, DRD4, and genes encoding for brain-derived neurotrophic factor, a protein that aids in the growth of new neurons. Variations in individual genes go a long way toward explaining why some children are affected more than others by their experiences growing up. Recent epigenetic studies even indicate that whether genes are turned on or off in response to particular environmental exposures can vary dramatically from one person to the next.

So far scientists have primarily examined genetic variations thought to be linked to psychiatric conditions, which are typically viewed as conferring serious lifetime risks. But newer work suggests that these genotypes are not just potential liabilities. They seem to demarcate greater and lesser plasticity, allowing for a wider range of possible behaviors and developmental outcomes. As in our rewrite of Achilles' destiny, these so-called vulnerability genes can be assets.

Consider what are known as short alleles of 5-HTTLPR, a genetic variation associated with depression. In 2014 developmental psychologist Grazyna Kochanska of the University of Iowa and her colleagues found that with positive parenting, carriers appeared to be the least likely to succumb to future substance abuse. The team monitored interactions between parents and kids, ages two to 10, in 100 families and then asked the children how they felt about drug use. The 10-year-olds with at least one short copy of 5-HTTLPR who also scored high on a measure of anger, used to gauge temperament, had the most antidrug attitudes of all the children in the study, provided they had been parented well. In keeping with the for better and for worse pattern, though, they were the most open to experimentation after more problematic parenting. For kids carrying only long alleles of 5-HTTLPR, there was no clear relation between their parenting and their views on drug use.

Teens with short 5-HTTLPR alleles also showed the greatest capacity for good behavior in a 2011 study led by psychologist Gene H. Brody of the University of Georgia. He and his colleagues genotyped 461 African-American adolescents in Georgia and interviewed them between the ages of 15 and 17 about their perceptions of racial bias. Again, out of everyone they evaluated, the boys carrying either one or two short versions of 5-HTTLPR—inherited from one or both parents—displayed the least antisocial behavior and aggression if they had experienced very little discrimination in their lives. Male carriers had the most conduct problems, however, if they reported perceiving a lot prejudice. (Why the girls did not follow this pattern remains unclear.)

Similar results are coming in for carriers of a long form of DRD4, regarded as a “risk allele” for its supposed connection to attention-deficit/hyperactivity disorder. In 2008 developmental scientist Marian J. Bakermans-Kranenburg of Leiden University in the Netherlands and her co-workers recruited 157 toddlers exhibiting behavioral problems—whining, biting, violent tantrums—and coached their mothers to use more sensitive disciplining techniques, including time-outs and praising good behaviors. Among the kids who possessed at least one copy of the long DRD4 allele, the intervention dramatically curtailed both aggression and disobedience. And this result was even more pronounced when the investigators restricted their focus to the mothers who gained the most from the training. Were these more responsive mothers also carriers of the long DRD4 allele? It certainly seems possible because their highly responsive, long allele–carrying children would have had to inherit it from at least one parent.

A more recent study also showcases the potential upside for long DRD4 allele carriers when they receive good parenting. Between 2004 and 2010 Brody and fellow University of Georgia psychologist Steven R. H. Beach ran an intervention called Adults in the Making (AIM), in which nearly 300 rural African-American youths and their families—some of whom were part of the 5-HTTLPR study mentioned earlier—attended six consecutive weekly group meetings. Parents learned how to provide emotional support and encourage responsible decision making, and kids learned how to plan for the future. When Brody, Beach and their colleagues later analyzed the results, they found that AIM proved most successful in countering substance abuse for teens with DRD4 long alleles. About two years after the program ended, the long allele–carrying kids who participated in the classes were still the least likely of all the subjects to drink or take recreational drugs.

Establishing a yardstick

Of course, we will never be able to identify environmentally sensitive children by looking at only one or another “candidate” gene. There are likely scores of genes involved, well beyond the few described here. Thus, to try to capture a snapshot of individual degrees of susceptibility, scientists are increasingly creating measures called polygenic scores or indices. These genetic yardsticks assign scores to children that reflect the presence or absence of multiple plasticity alleles. So far the results are encouraging.

In their AIM analysis, Brody and Beach created a polygenic plasticity index that included DRD4 and two additional genes. They discovered that the program's benefits—that is, decreases in drug and alcohol use—were greatest for those scoring highest on the index. And consistent with for better and for worse thinking, kids scoring just as high but in a control group actually drank the most alcohol. My colleagues and I designed another index, based on five genes, to evaluate 1,586 adolescents in the NICHD-funded Add Health project, the largest longitudinal study of adolescent health ever undertaken. We, too, found the higher the polygenic score, the more strongly parenting styles predicted self-control, at least among boys.

And recent work published in 2014 by April S. Masarik of the University of California, Davis, and her co-workers also used five genes to measure environmental sensitivity. Among their highest scorers, those with the most successful romantic relationships as adults enjoyed the most engaged parents as teens; those with the worst romances had experienced more hostile caregiving.

So let us imagine that one day in the not too distant future, even broader indices make it possible to confidently single out and assign relative scores to environmentally sensitive children. Should we selectively give the children above a certain cutoff extra nurturing and enrichment to ensure that they reach their full potential? Do we exclude the kids less likely to see any gains? I have discussed the idea with many friends and colleagues and know that those who value equity over efficacy object to the notion of treating children differently based on temperament or genetic makeup.

But many existing initiatives—for example, the government's Head Start program, launched more than half a century ago to provide early education to three- and four-year-olds from low-income families—receive lukewarm reviews at best. Advocates tend to exaggerate the positive findings, and critics emphasize the negative ones. These costly programs would probably show more consistently positive results if they especially targeted the subset of children highly sensitive to environmental influences. If we could identify those children, why shouldn't we? What is ethical, after all, about providing services, using taxpayers' money, to kids who will not be helped by them? Would we ever dispense an expensive medicine indiscriminately if we knew it would aid only some people, especially if in doing so, it meant possibly failing to provide the treatment to those most likely to get well?

Let me say clearly that even if we can eventually pinpoint the most responsive children, it would not mean abandoning the rest. Every child deserves a decent quality of life, no matter the long-term consequences—or lack thereof. Furthermore, money saved by restricting interventions to those most likely to benefit from them should be used to explore different and conceivably radical new programs for everyone else. After all, we do not know if the children who seem unmoved by our current initiatives would be similarly unaffected by a different approach. Our ultimate goal should not be to save money but to direct our resources more wisely.