The arrival of puberty is a bodily event influenced by psychological forces. The most potent of these forces is stress: decades of research have demonstrated that a stressful childhood accelerates reproductive development, at least as measured by menarche, or the first menstrual cycle. For instance, girls growing up with fathers who have a history of socially deviant behavior tend to undergo puberty a year earlier than those with more stable fathers, while girls who have been maltreated (primarily because of physical or sexual abuse) begin menarche before those who have not. One study even found that Finnish girls evacuated from their homeland during WWII – they had to endure the trauma of separation from their parents – reached puberty at a younger age and had more children than those who stayed behind.
There’s a cold logic behind these correlations. When times are stressful, living things tend to devote more resources to reproductive development, as they want to increase the probability of passing on their genes before death. This leads to earlier puberty and reduced investment in developmental processes less directly related to sex and mating. If nothing else, the data is yet another reminder that early childhood stress has lasting effects, establishing developmental trajectories that are hard to undo.
But these unsettling findings leave many questions unanswered. For starters, what kind of stress is the most likely to speed up reproductive development? Scientists often divide early life stressors into two broad categories: harshness and unpredictability. Harshness is strongly related to a lack of money, and is typically measured by looking at how a family’s income relates to the federal poverty line. Unpredictability, in contrast, is linked to factors such as the consistency of father figures inside the house and the number of changes in residence. Are both of these forms of stress equally important at triggering the onset of reproductive maturation? Or do they have different impacts on human development?
Another key question is how this stress can be buffered. If a child is going to endure a difficult beginning, then what is the best way to minimize the damage?
These questions get compelling answers in a new study by a team of researchers from four different universities. (The lead author is Sooyeon Sung at the University of Minnesota.) Their subjects were 492 females born in 1991 at ten different hospitals across the United States. Because these girls were part of a larger study led by the National Institute of Child Health and Human Development, Sung. et al. were able to draw on a vast amount of relevant data, from a child’s attachment to her mother at 15 months to the fluctuating income of her family. These factors were then tested against the age of menarche, as the scientists attempted to figure out the psychological variables that determine the onset of puberty.
The first thing they found is that environmental harshness (but not unpredictability) predicts the timing of the first menstrual cycle. While this correlation is limited by the relatively small number of impoverished families in the sample, it does suggest that not all stress is created equal, at least when it comes to the acceleration of reproductive development. It’s also evidence that poverty itself is stressful, and that children raised in the poorest households are marked by their scarcities.
But the news isn’t all terrible. The most significant result to emerge from this new paper is that the effects of childhood stress on reproductive development can be minimized by a secure mother-daughter relationship. When the subjects were 15 months old, they were classified using the Strange Situation procedure, a task pioneered by Mary Ainsworth in the mid-1960s. The experiment is a carefully scripted melodrama, as a child is repeatedly separated and reunited with his or her mother. The key variable is how the child responds to these reunions. Securely attached infants get upset when their mothers leave, but are excited by her return; they greet her with affectionate hugs and are quickly soothed. Insecure infants, on the other hand, are difficult to calm down, either because they feign indifference to their parent or because they react with anger when she comes back.
Countless studies have confirmed the power of these attachment categories: Securely attached infants get better grades in high school, have more satisfying marriages and are more likely to be sensitive parents to their own children, to cite just a few consistent findings. However, this new study shows that having a secure attachment can also dramatically minimize the developmental effects of stress and poverty, at least when measured by the onset of puberty.
Love is easy to dismiss as a scientific variable. It’s an intangible feeling, a fiction invented by randy poets and medieval troubadours. How could love matter when life is sex and death and selfish genes?
And yet, even within the unsparing framework of evolution we can still measure the sweeping influence of love. For these children growing up in the harshest environments, the security of attachment is not just a source of pleasure. It's their shield.
Sung, Sooyeon, Jeffry A. Simpson, Vladas Griskevicius, I. Sally, Chun Kuo, Gabriel L. Schlomer, and Jay Belsky. "Secure infant-mother attachment buffers the effect of early-life stress on age of menarche." Psychological Science, 2016.