annual report 2011 - Office for Research - Northwestern University

Christopher Kuzawa
Weinberg College of Arts and Sciences and
Institute for Policy Research
How Human Fathers Are Unique
Humans are among the rare mammals whose fathers are
directly involved in rearing and helping care for their young.
In what ways has this unusual strategy left its mark on
human biology and behavior? One fascinating example is the
hormone testosterone, which tends to be less abundant in
fathers than in nonfathers. This long-standing observation
is consistent with what has been observed in bird species
in which males are heavily involved with protecting nests,
incubating eggs, and feeding newborn chicks. Conventional
wisdom suggested that a drop in testosterone in human
fathers could be the body’s way of helping dads shift
priorities from wooing mates to parenting. Past research,
however, could not determine what is cart and what is horse
in these relationships: Does becoming a father lead to a drop
in a man’s testosterone? Or, alternatively, are men who have
lower testosterone to begin with just more likely to become
fathers? Because all past studies had measured men at single
time points, the answer was not clear.
Christopher Kuzawa, anthropology, has worked for 15 years
with a unique study in the Philippines that has followed a
large cohort of men and women over long time periods,
allowing him to address questions about human biology and
health with longitudinal data. In a recent National Science
Foundation–funded collaboration with his PhD student
and colleagues at Northwestern and abroad, Kuzawa’s team
measured testosterone levels in a large sample of young
men who were not fathers and then followed up with a
second hormone assessment five years later. In the five
years between measurements, many formerly single men
had entered stable relationships and had become fathers.
Kuzawa’s team found that men who were destined to become
The drop in evening testosterone during a 5-year period was largest among Filipino men
who were single non-fathers at baseline (2005) but who had entered stable relationships and
become fathers by follow-up in late 2009. Changes in testosterone were much smaller among
men who did not experience a change in relationship or fatherhood status during this 5 year
period. *denotes a statistically signi�cant change (p