Although we tend to overlook it, we humans are animals and, although we sometimes don't realize it, we act just like our wilder relatives. This was highlighted in a lecture today by Daniel Nettle of Newcastle University, who discussed how early-life environment affects reproductive strategies in human females. His research was inspired, in part, by previous work showing that female monkeys with more disrupted childhoods were later more interested in holding infants when given a choice between an infant and a model. In other words, these females seemed more primed to engage in reproductive behaviors than similar-aged monkeys who had experienced a normal childhood.
These results jived with several findings that have been found, separately, in studies of human reproductive behavior and stressful childhood experiences. For instance, adolescent females are likely to have an earlier first period if they are born with a low birth weight, raised without a father in the home, or are adopted; teenage pregnancy has also been linked with father absence, as well as several other early-life circumstances.
In order to tie these trends together, and see whether the monkey study showed patterns that are also present in humans, Nettle and colleagues utilized a detailed long-term database from the UK. Data collection first began in 1958 for a total of 17,416 individuals who were born that year; researchers followed up with the study participants periodically, most recently in 2004, at which point 11,939 participants remained in the study. The current study examined 4,533 females from this group in order to determine whether age at first pregnancy was affected by socioeconomic status during childhood, age of their own mother at the time of their birth, adverse experiences early in life, and emotional or behavioral issues during childhood (as previously assessed by their teachers).
Each of these variables had a significant effect on age at first birth, though the size of the effect was consistently small--often influencing age at first pregnancy by just 6 months or a year. Women who were breast-fed more started their first pregnancy later. However, women who were separated from their mothers as children gave birth earlier, and the strength of the effect was dependent on the length of separation. The only exception to this trend was among women who were separated from their mothers for more than 2 years--a situation generally linked with removal to a foster home or adoption, both circumstances in which a surrogate mother was present. First pregnancy was also earlier in women whose fathers were absent, or at least played less of a role in their upbringing; likewise, women who experienced the stress of moving as children also gave birth sooner.
The researchers performed a second layer of analysis in which they tallied how many of these stressors each woman was exposed to. It turns out that this number is highly correlated with age at first childbirth, such that women with fewer overall stressors gave birth later, and women with more stress gave birth earlier.
What drives these trends, and why? The researchers aren't sure of the answers to either of those questions just yet. However, they suspect these patterns are influenced by the hypothalamic-pituitary-adrenal axis, which controls reactions to stress and regulates a huge range of body processes--including immune function, sexuality, digestion, mood and emotions, and energy expenditure, among others. Nettle did not comment on the evolutionary advantage of these patterns--in monkeys or in humans--but I suspect it might be a good coping mechanism for animals who are living (or who think they are living) in a harsh or unpredictable environment: Producing your young earlier would certainly be one way of making sure that you leave a copy of your genes behind as some point, no matter what curve balls life throws your way.
Regardless, it's always interesting to see some research that shows humans acting like other animals, even subconsciously. It's fascinating, but a little scary--a reminder that sometimes we are just puppets dancing on the ends of strings manipulated by our genes!
Thanks to the following websites for providing the photos used in this post: