Comparing Evolutionary Models of Sexual Initiation

by Kristin Snopkowski & John Ziker


Life history theory examines how organisms allocate energy throughout the lifespan.  While it was originally developed in biology, more recently a large number of researchers have begun to use life history theory as a way to examine phenotypically plastic responses to early life factors in humans.  This research has examined how family dynamics and stressors earlier in life influence ‘life history events,’ such as pubertal timing, age at first reproduction, age at sexual initiation, and more rarely, investment in offspring.

In this research, we tested a variety of different evolutionary hypotheses that have been proposed – frequently under the theoretical umbrella of life history theory – to see which were best at predicting sexual initiation among a sample of Canadian adolescents who were followed longitudinally since birth (over eight waves of data collection).  The National Longitudinal Survey of Children and Youth, a survey conducted by Statistics Canada, provides a wealth of information about children and their families.  This allowed us to examine a variety of different factors that have been predicted to influence the timing of life history events (although, our analysis only examined sexual initiation).  This includes factors such as: childhood psychosocial stressors, such as parenting behaviors, socioeconomic status, stressful events; paternal investment indicators, including contact with father, parental divorce, emotional closeness with father; mortality cues, for example, health of caregivers, adolescent chronic illness; unpredictability, such as changes in childcare and household moves; social support, for instance, adolescent social support other than peers; prenatal factors, including birth health and gestational diabetes; and intergenerational conflict indicators, measured as having half siblings.

We utilized a model selection procedure where we minimized AIC scores to predict whether adolescents had engaged in consensual sexual intercourse by the ­eighth wave of data collection (age 14/15). After identifying the predictors that minimized the AIC score for each variable set, we ran all combinations of models (since we had eight sets of variables, this led to 256 different combinations).  We then examined all models within three of the best AIC score (indicative of having a similar level of statistical support) and calculated the importance score for each variable set.

“The lack of an association between paternal investment variables (frequently discussed as ‘father absence’) was surprising given the frequency with which it has been found in other contexts”

The results showed that psychosocial stressors (either early-life or adolescent), mortality cues, and intergenerational conflict variables were included in all of the top models. Prenatal factors were included in the top five models. Social support and unpredictability were common across top models (but not included in all models).  There was greater support for early-life psychosocial stressors over stressors during adolescence.  Paternal investment variables were not included in any of the top models.  Importance scores mirrored the AIC results: extrinsic mortality cues had the most importance, followed by intergenerational conflict, early childhood psychosocial stressors and prenatal factors. Social support and unpredictability predictors followed, with importance scores between 0.58 and 0.54.  Finally, paternal investment has the lowest importance (0.11).

As previous research has found, there was good support for early-life psychosocial stressors and extrinsic mortality cues being associated with sexual initiation by age 14/15. As predicted social support, unpredictability, and prenatal factors were also associated with sexual initiation.  Novel results of this study include the role of half-siblings and its association with sexual initiation, as predicted by intergenerational conflict models (a few recent papers have also found evidence of this).  The lack of an association between paternal investment variables (frequently discussed as ‘father absence’) was surprising given the frequency with which it has been found in other contexts (mostly western contexts, like the one we examined in this study).  It is possible that intergenerational conflict factors confound the effect of parental divorce, so it is important to include both factors in future models to detangle their effects.



Read the paper: “Sexual initiation among Canadian youth: A model comparison approach of evolutionary hypotheses shows greatest support for extrinsic mortality cues, intergenerational conflict, and early life psychosocial stressors