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In species in which males care for young, testosterone (T) is often high during mating periods but then declines to allow for caregiving of resulting offspring. This model may apply to human males, but past human studies of T and fatherhood have been cross-sectional, making it unclear whether fatherhood suppresses T or if men with lower T are more likely to become fathers. Here, we use a large representative study in the Philippines (n = 624) to show that among single nonfathers at baseline (2005) (21.5 ± 0.3 y), men with high waking T were more likely to become partnered fathers by the time of follow-up 4.5 y later (P < 0.05). Men who became partnered fathers then experienced large declines in waking (median: -26%) and evening (median: -34%) T, which were significantly greater than declines in single nonfathers (P < 0.001). Consistent with the hypothesis that child interaction suppresses T, fathers reporting 3 h or more of daily childcare had lower T at follow-up compared with fathers not involved in care (P < 0.05). Using longitudinal data, these findings show that T and reproductive strategy have bidirectional relationships in human males, with high T predicting subsequent mating success but then declining rapidly after men become fathers. Our findings suggest that T mediates tradeoffs between mating and parenting in humans, as seen in other species in which fathers care for young. They also highlight one likely explanation for previously observed health disparities between partnered fathers and single men.
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Longitudinal evidence that fatherhood decreases
testosterone in human males
Lee T. Gettler
a,b,1,2
, Thomas W. McDade
a,b
, Alan B. Feranil
c
, and Christopher W. Kuzawa
a,b,1,2
a
Department of Anthropology, and
b
Cells to Society, Center on Social Disparities and Health, Institute for Policy Research, Northwestern University, Evanston,
IL 60208; and
c
Ofce of Population Studies Foundation, University of San Carlos, Cebu City 6000, Philippines
Edited by A. E. Storey, Department of Psychology, Memorial University of Newfoundland, St. Johns, NF, Canada, and accepted by the Editorial Board July 28,
2011 (received for review May 10, 2011)
In species in which males care for young, testosterone (T) is often
high during mating periods but then declines to allow for
caregiving of resulting offspring. This model may apply to human
males, but past human studies of T and fatherhood have been
cross-sectional, making it unclear whether fatherhood suppresses
T or if men with lower T are more likely to become fathers. Here,
we use a large representative study in the Philippines (n= 624) to
show that among single nonfathers at baseline (2005) (21.5 ±0.3 y),
men with high waking T were more likely to become partnered
fathers by the time of follow-up 4.5 y later (P<0.05). Men who
became partnered fathers then experienced large declines in wak-
ing (median: 26%) and evening (median: 34%) T, which were
signicantly greater than declines in single nonfathers (P<0.001).
Consistent with the hypothesis that child interaction suppresses T,
fathers reporting 3 h or more of daily childcare had lower T at
follow-up compared with fathers not involved in care (P<0.05).
Using longitudinal data, these ndings show that T and reproduc-
tive strategy have bidirectional relationships in human males, with
high T predicting subsequent mating success but then declining
rapidly after men become fathers. Our ndings suggest that T
mediates tradeoffs between mating and parenting in humans, as
seen in other species in which fathers care for young. They also
highlight one likely explanation for previously observed health
disparities between partnered fathers and single men.
challenge hypothesis
|
human evolution
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hormones and behavior
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paternal care
|
reproductive ecology
In male mammals, testosterone (T) stimulates the development
and maintenance of traits and behaviors that contribute to
male mating effort, including musculature, libido, conspecic
aggressivity, and courtship (14). Although these T-driven traits
factor into mating success, male reproductive tness in some
avian and mammalian species also depends on contributions to
offspring care (5, 6). Because time and energy are nite (7),
males in these species often face tradeoffs between conicting
behaviors related to mating and parenting. Adjustment of T
production has been proposed as a physiological mechanism
underlying this tradeoff, with males who focus on mating effort
predicted to maintain elevated T, whereas males who cooperate
with a female partner and invest in parental care should reduce T
production (6, 8). This model is well supported by data from
a variety of avian species (6, 8), but evidence for its applicability
to mammalian species in which males provide direct care is
mixed (9). It is presently unclear whether T mediates the tradeoff
between mating and parenting effort in human males, who often
express paternal care facultatively.
Humans have been described as serial monogamists who fre-
quently engage in one or more long-lasting partnerships with
females during reproductive life spans that last several decades
(1012). Humans are one of the few mammalian species in which
paternal care is relatively common, with fathers often helping to
raise multiple overlapping offspring who are dependent well into
their second decade of life (5, 1315). If T contributes to human
male reproductive strategy, high initial T should enhance a mans
mating success, but men who have succeeded in securing a mate
and/or fathering a child should then down-regulate T, particu-
larly if they frequently care for their children (6, 8, 13).
Past human work provides indirect support for these expect-
ations largely using cross-sectional data. Multiple studies have
shown that partnered men have lower T compared with single
men (16, 17), and a large 10-y study of US servicemen found that
T decreased in men who married during the study period (18). In
comparisons of men varying in both relationship and parenting
status, partnered fathers have been shown to have the lowest T
overall, differing signicantly from single nonfathers in some
populations (1921), including the present study population (22,
23). There is also increasing evidence that caregiving predicts
which fathers have lowest T (20, 22, 24). Although these cross-
sectional correlations are generally consistent with the presumed
suppressive effect of partnering and fatherhood on T production
(a stateeffect), such ndings could alternatively result if men
with low T are more likely to become partnered or fathers (a
traiteffect) (25). However, to date, no human study has
monitored hormonal changes longitudinally as single nonfathers
transition into stable partnerships and become fathers.
To clarify the role of T in human male reproductive strategy,
we draw on data and biological samples collected in a large
sample of men participating in the Cebu Longitudinal Health
and Nutrition Survey (CLHNS), a representative 1-y birth cohort
study begun in the Philippines in 1983. In addition to longitu-
dinal questionnaire data, we collected saliva samples for T
measurement at waking (AM) and before bed (PM) in all par-
ticipants (n= 624) when they were 21.5 (±0.3) y of age (base-
line) and again when they were 26.0 (±0.3) y of age (follow-up).
All participants live in or around Cebu City, the Philippines,
where it is common for fathers to be involved in day-to-day care
of their children (22). Focusing on the subsample of men who
were single nonfathers at baseline (n= 465), we tested the hy-
potheses that (i) men with higher baseline T would have greater
mating success as indicated by being in a stable partnership
(married/cohabitating) and/or becoming a father by the time of
follow-up and (ii) these newly partnered new fathers would
subsequently show a greater decrease in T than men who
remained single nonfathers. We also tested the hypothesis that
(iii) fathers who reported spending more time in childcare would
have lower T at follow-up than fathers who reported spending
Author contributions: L.T.G., A.B.F., and C.W.K. designed research; L.T.G., T.W.M., A.B.F.,
and C.W.K. performed research; L.T.G. and C.W.K. analyzed data; and L.T.G., T.W.M.,
A.B.F., and C.W.K. wrote the paper.
The authors declare no conict of interest.
This article is a PNAS Direct Submission. A.E.S. is a guest editor invited by the Editorial
Board.
See Commentary on page 16141.
1
L.T.G. and C.W.K. contributed equally to this work.
2
To whom correspondence may be addressed. E-mail: lgettler@u.northwestern.edu or
kuzawa@northwestern.edu.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1105403108/-/DCSupplemental.
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less time in childcare, as would be consistent with a direct sup-
pressive role of caregiving on T among fathers.
In observational research, correlated nonmeasured factors can
inuence both predictor and outcome variables, and thus lead to
confounded associations. Here, we use an econometric change
model that minimizes the likelihood of such confounding, be-
cause any permanent or stable factors that differ among men but
that have not changed in the follow-up period are included
within the error term of both regression models, and thus are
eliminated as potential inuences on any change in T experi-
enced during the period of follow-up (26).
Results
Table 1 summarizes sociodemographic and biological charac-
teristics for the full sample and also stratied on a median split of
AM T measured at follow-up (2009). When the full sample was
considered, all men showed a modest but signicant decrease in
both AM and PM T between baseline and follow-up, consistent
with age-related declines documented previously (27). Men with
high T tended to have completed more years of education,
whereas men with low and high T did not differ in anthropo-
metric measures. Consistent with the results presented below,
a greater proportion of partnered men and fathers had T below
the median.
We rst used logistic regression to test whether T at baseline
predicted reproductive status at follow-up (n= 465; Table S1).
Consistent with our hypothesis, single nonfathers with greater
AM T at baseline were more likely to become newly partnered
[odds ratio (OR) = 1.20, P= 0.044] and newly partnered new
fathers (OR = 1.21, P= 0.048) by the time of follow-up in 2009.
We next tested whether men who became newly partnered
new fathers during the period of follow-up experienced a greater
decline in T relative to men who remained single nonfathers
between baseline and follow-up. Consistent with this hypothesis,
men who were both newly partnered and new fathers showed the
largest declines in AM and PM T between 2005 and 2009 (Fig. 1
Aand B), and their declines in both AM (median: 26%) and
PM (median: 34%) T were signicantly greater than the
modest age-related declines in AM (median: 12%) and PM
(median: 14%) T observed among single nonfathers (P<0.001,
n= 465; Fig. 1Band Tables S2 and S3). Newly partnered men
who remained nonfathers at follow-up showed declines in AM
(median: 10%) and PM (median: 32%) T (Fig. 1A) that were
not signicantly different from those of single nonfathers [ab-
solute change in AM T (ΔAM T), P= 0.499; ΔPM T, P= 0.167;
Fig. 1Band Tables S2 and S3]. Effects of change in partnering
and fatherhood status on ΔAM and ΔPM T were not sub-
stantially affected after adjusting for self-reported psychosocial
stress and sleep quality, neither of which signicantly predicted
ΔT (all P>0.3; Tables S2 and S3). In addition, men who were
partnered fathers in 2005 and who already had low T at baseline
showed only slight within-group declines in both ΔAM T (me-
dian: 7%, P= 0.048; n= 83) and ΔPM T (median: 3%, P=
0.374) by follow-up in 2009 (Fig. 1Aand Table S4).
We next tested whether the large decrease in T among new
fathers was contingent on the age of a mans youngest child.
Although all new fathers, regardless of their youngest childs age,
experienced a signicant reduction in AM and/or PM T com-
pared with nonfathers (Fig. 2 and Tables S5 and S6), fathers with
newborns (1 mo old or less) at the time of follow-up hormone
assessment showed signicantly greater declines in AM (P=
0.023) and PM (P= 0.003) T compared with fathers whose
youngest child was older than 1 y of age, which was not ac-
counted for by reports of psychosocial stress, sleep quality, or
involvement in caregiving (Tables S7 and S8). Men with new-
borns also differed signicantly for ΔAM T compared with men
with infants between 1 mo and 1 y of age (P= 0.007).
Finally, we evaluated whether men who were fathers at follow-
up (n= 312) varied in T based on their self-reported in-
volvement in childcare in 2009, controlling for sleep quality,
psychosocial stress, and number of children. Men reporting 13h
of daily childcare had signicantly lower AM T compared with
fathers reporting not being involved with care (Fig. 3), whereas
fathers reporting the highest involvement in childcare (3 h or
more per day) showed signicantly lower values of both AM and
PM T compared with men reporting no care (Fig. 3). Consistent
with the hypothesis that childcare suppresses T, among men who
began as nonpartnered nonfathers at baseline (n= 162), time
Fig. 1. (A) Within-group changes in AM and PM T values between 2005 and 2009. Mean values of T, adjusted for time of saliva collection and usual wake
time (AM), were compared using paired ttests. Group 1 (n= 83) comprised men who were partnered and fathers in 2005 and 2009. Group 2 (n= 257)
comprised men who were not partnered in 2005 and 2009 and were never fathers. Group 3 (n= 46) comprised men who became partnered between 2005 and
2009 and were never fathers. Group 4 (n= 162) comprised men who became partnered and were rst-time fathers between 2005 and 2009. *P<0.05; **P<
0.01,
P<0.0001. Error bars indicate SEM. P, partnered. (B) Between-group changes in AM and PM T values between 2005 and 2009 based on partnering and
parenting status. Group descriptions are as in A. Values were adjusted for time of saliva collection and usual wake time (AM) and are derived from regressing
the change in T on changes in partnering and parenting status, with group 2 as the comparison group, controlling for sleep quality and psychosocial stress
(Tables S2 and S3). Partnered fathers are included for visual comparison but were not part of the regression analyses. ***P<0.001. Error bars indicate SEM.
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spent in childcare as fathers (at follow-up) was not predicted by
either AM T [ordered logistic OR = 1.12, 95% condence in-
terval (CI): 0.841.48; P= 0.441] or PM T (OR = 0.89, 95% CI:
0.671.18; P= 0.417) measured at baseline.
Discussion
Among the men in our sample who were single nonfathers as
young adults, those with higher waking T were more likely to
have become a partnered father by the time of follow-up. Once
these men entered stable partnerships and became new fathers,
they subsequently experienced a large decline in T, which was
greater than the comparably modest declines seen in single
nonfathers during the same period. Finally, fathers who were
most involved in childcare had lower T compared with fathers
who did not participate in care. Using longitudinal data, these
results demonstrate that high T not only predicts mating success
(i.e., partnering with a female and fathering a child) in human
males but that T is then greatly reduced after men enter stable
relationships and become fathers. The nding that high in-
volvement in childcare was associated with low T measured at
follow-up but was not related to baseline T supports the hy-
pothesis that direct care of dependent offspring suppressed T
among the fathers in our sample (20, 22). Our ndings suggest
that human males have an evolved neuroendocrine architecture
that is responsive to committed parenting, supporting a role of
men as direct caregivers during hominin evolution (13, 14, 21).
Our results provide longitudinal evidence that high T predicts
subsequent mating success in human males. Although we did not
measure the behavioral or physical pathways linking T with
mating success in this analysis, T has previously been shown to
bolster traits related to mating effort and attractiveness, such as
musculature (1, 28, 29), motivation to win during competition
(30), and pursuit of social dominance (2, 31). Men with higher T
have also been shown to have physical attributes deemed at-
tractive by females and to have more recent and lifetime sexual
partners (3234). Although families traditionally played a pri-
mary role in arranging courtship and marriage in the Philippines,
courting in recent decades has gradually moved toward males
and females meeting independent of familial control (35). Mens
romantic prospects may thus be increasingly contingent on male-
male competition, particularly in social and economic domains
(36). This trend has likely increased the potential for high T to
factor into male mating success.
Although helpful in securing mates, many T-stimulated
behaviors may conict with partnership stability and parenting
(4, 33). Indeed, men with higher T have been shown to be more
likely to have marital problems and to be divorced (4, 18),
whereas men with lower T have been found to spend more time
with their wives (21). In an experimental setting, men with
greater T also reported feeling less sympathy or need to respond
to infant cries compared with men with lower T (37). Although
prior cross-sectional studies have led to speculation that father-
hood decreases T in human males (19, 22, 24), our longitudinal
results demonstrate that fatherhood causes T to decline and
remain low. These ndings were not substantively changed when
covariates (psychosocial stress and sleep quality) that might be
expected to mediate the relationship between fatherhood/
marriage and T were included in models and are consistent with a
previous longitudinal report that men who were married experi-
enced decreased T (18).
We also found that T at follow-up was lowest among fathers
reporting more hours spent in childcare. Although this nding
could result if men with low T at baseline were more likely to get
involved in childcare, instead, we found that childcare in-
volvement was unrelated to T at baseline. Familial composition
was not a confounding inuence on the relationships that we
documented, which is consistent with previous research from
Cebu reporting that fathers did not alter their childcare partici-
pation based on their number of children (38). Together, these
ndings provide longitudinal support for the hypothesis that
interacting with a dependent child suppresses T (20, 22, 24). In
prior research conducted in two neighboring cultural groups in
Tanzania, fathers in the population in which paternal care is the
cultural norm had lower T, whereas this was not found among
fathers in the group in which paternal care is absent (20). In
a study of a polygynous Senegalese society, it was found that
fathers who were highly invested in their children, as reported by
the childrens mothers, had lower T compared with fathers who
were less invested (24). Lower T has also been associated with
nurturing behaviors among fathers (37, 39).
Fig. 2. Between-group changes in AM and PM T values between 2005 and
2009 with fathers stratied by child age. Values are adjusted for time of
saliva collection and usual wake time (AM) and are derived from regressing
the change in T on fatherhood, stratied by child age, with men who were
not fathers in 2005 and 2009 as the comparison group, and controlling for
sleep quality and psychosocial stress (Tables S5 and S6). Fathers of newborns
were men whose youngest child was in the perinatal period [1 mo (m) old or
less]. Fathers of infants were men whose youngest child was older than 1 mo
(m) but 1 y (yr) old or less. ^P<0.10; **P<0.01; ***P<0.001. Error bars
indicate SEM.
Fig. 3. 2009 AM and PM T values among fathers varying in daily physical
childcare. Values were derived from regressing T on daily paternal caregiv-
ing, controlling for time of saliva collection, usual wake time (AM), sleep
quality, psychosocial stress, and number of children, with fathers who
reported no involvement in childcare as the comparison group. No care (n=
34), 01h(n= 37), 13h(n= 139), 3+ h (n= 102). Regression models were
calculated with robust SEs.
a
P= 0.020;
b
P= 0.044;
c
P= 0.015. AM model: R
2
=
0.047; PM model: R
2
= 0.046. Error bars indicate SEM.
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Although all new fathers had lower T than men remaining
single nonfathers, our results also suggest that fathers of new-
borns (1 mo old or less) experienced a large transient decline in
T that was signicantly greater than that of fathers whose
youngest child was older than 1 mo of age. This nding is con-
sistent with a previous cross-sectional study in which fathers of
newborns were found to have lower T compared with a group of
expectant fathers, whose T had been measured during their
partnerspregnancies (39). In another study in which expectant
fathers were sampled for T multiple times during their partners
pregnancies and after the women gave birth, those men with high
T during the pregnancy showed a signicant decline in the rst
week after birth (40). Viewed alongside these past ndings, the
steep transient T decrease that we document among fathers with
newborns could indicate an anticipatory psychological compo-
nent to mens T decline around the time of birth of their chil-
dren. Alternatively, our control variables may not fully capture
the scope of sleep disruption and psychosocial adjustments that
accompany a familys accommodation of a newborn baby, which
could contribute to this large short-term T decline in the post-
partum period. Taken together, our ndings suggest that antic-
ipatory or other effects unique to the immediate period of
parturition are likely additive to the more sustained effects of
caregiving in suppressing paternal T.
Our results are consistent with ndings from many bird spe-
cies, among which fathers often show declines in T during
periods in which they help raise young (6, 41). Relevant ndings
from other mammals are less consistent, with fathers having
lower T in some (4245) but not all (4648) mammalian species
in which fathers assist with offspring care. Analogies to bird and
other mammalian species are somewhat constrained because
humans are not seasonal breeders and have signicantly longer
interbirth intervals (34 y) and slowly developing dependent
offspring (49, 50). Moreover, human malespredisposition to-
ward paternal care is likely a derived trait that emerged during
hominin evolution (1315, 51, 52). Thus, compared with other
species with paternal care, mens T might decrease with father-
hood and then remain low over a longer and more sustained time
period corresponding to the slow life history of humans and the
prolonged dependency of offspring (50, 53, 54).
There is considerable interest in the health differentials be-
tween fathers and single men (55), and it is often reported that
married men and fathers have lower risk for certain diseases and
mortality (5658). Our ndings suggest that fathers are likely
exposed to lower levels of T throughout much of their prime
reproductive years, which could contribute to some of these
health differentials. For instance, high T may increase risk for
prostate cancer and adverse cholesterol proles, and high T has
also been linked to risk-taking behaviors that can affect mens
health, such as drug and alcohol use and promiscuity (33, 59, 60).
Our nding that men who end up as fathers tend to have higher
T to begin with also suggests that some of the benets of low T
among fathers could be offset by higher T exposure among these
men before becoming fathers, which could hinder efforts to
identify the health impacts of being a partnered father. Thus, our
ndings point to likely health effects of fatherhood and also
underscore some of the complexities of this exposure. The large
reductions in circulating T among the new fathers in our sample
provide a strong rationale to investigate linkages between fa-
therhood status and risk for diseases related to T exposure.
In sum, our results provide longitudinal conrmation that T
exhibits a bidirectional relationship with reproductive strategy in
human males. Single nonfathers with higher T at baseline were
more likely to be partnered fathers 4.5 y later. After becoming
partnered fathers, these men experienced dramatic reductions
in both waking and evening T, which were substantially greater
than the age-related declines observed in single nonfathers. Our
nding that caregiving fathers had lower T than fathers who did
not invest in care supports the hypothesis that father-child in-
teraction likely contributes to suppressed paternal T among fathers.
Table 1. Sample characteristics stratied on low and high follow-up (2009) AM T (n= 624)
All (n= 624) Low AM T* (n= 316) High AM T* (n= 308)
Mean ±SD Mean ±SD Mean ±SD Pvalue
Demographic characteristics
Age, y 26.0 ±0.3 26.0 ±0.3 26.0 ±0.3 0.57
Highest grade completed 11.5 ±4.8 11.2 ±4.8 11.9 ±4.8 0.07
T values
AM T 2005, pg/mL 192.8 ±74.1 175.5 ±65.7 210.5 ±77.9 0.0001
AM T 2009, pg/mL
162.0 ±61.8 115.9 ±30.8 209.2 ±48.4 0.0001
PM T 2005, pg/mL 117.7 ±51.9 112.3 ±50.0 123.3 ±53.2 0.0001
PM T 2009, pg/mL
92.6 ±39.2 79.3 ±32.4 106.2 ±40.9 0.0001
Anthropometric measures
Body fat percentage 20.1 ±5.2 19.9 ±5.3 20.2 ±5.1 0.40
Body mass index, kg/m
2
22.7 ±3.6 22.5 ±3.8 22.9 ±3.4 0.23
Relationship characteristics
§
Partnered 2005 19.7% 20.6% 18.8% 0.59
Partnered 2009 54.0% 59.8% 48.1% 0.003
Duration of relationship
,y 3.7±2.4 3.6 ±2.4 3.8 ±2.4 0.45
Fatherhood characteristics
Father 2005 16.0% 15.8% 16.2% 0.89
Father 2009 50.0% 56.0% 43.8% 0.002
No. children
1.6 ±0.8 1.6 ±0.8 1.7 ±0.8 0.18
Age of oldest child
,y 3.4±2.2 3.2 ±2.2 3.6 ±2.2 0.16
*Test for signicant differences by median split of 2009 AM T (low AM T <155.6 pg/mL <high AM T); unpaired,
two-tailed ttest or χ
2
test.
2009.
Paired ttests comparing 2005 and 2009 AM and PM T (both P<0.0001).
§
Married/cohabitating.
Restricted to partnered men in 2009 (n= 337).
Restricted to fathers in 2009 (n= 312).
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These results point to an important role of the hypothalamic-
pituitary-gonadal axis as a mediator of the tradeoff between
investments in parenting and mating in human males, similar to
what is seen in other species in which paternal care is common.
They also add to evidence that human males have an evolved
neuroendocrine architecture shaped to facilitate their role as fa-
thers and caregivers as a key component of reproductive success.
Methods
Study Population. Data were collected in 2005 and 2009 as part of the CLHNS,
a representative population-based birth cohort study of mothers and their
infants born in 19831984 (61). Men (n= 624) were an average of 26.0 ±0.3
(SD) y old at the time of data and sample collection in 2009. Socioeconomic,
demographic, and behavioral data were collected during in-home interviews
administered by Cebuano-speaking interviewers (61). Men were classied as
partneredif they identied themselves as married or in a cohabitating
relationship (22). Fathers were dened as men who reported having one or
more biological children. Fathers of newborns were dened as men whose
youngest child was in the perinatal period (1 mo old or less). Fathers of
infants were dened as men whose youngest child was older than 1 mo of
age but less than 1 y old. Paternal caregiving was assessed via the question,
How much time do you usually spend providing physical care to your
children on a daily basis?with men grouped by no contact/0 min, less than
1h,13 h, and 3+ h.
Weight (kg), height (cm), and triceps skinfold thickness (mm) were
measured using standard anthropometric techniques (62). Body fat per-
centage was calculated from triceps skinfold thickness using body density
estimates and a body composition predictive equation (63). The body mass
index was calculated as the ratio of weight (kg)/height (m
2
). Self-reported
psychosocial stress in the month preceding sampling was quantied via
a modied version of the 10-item Perceived Stress Scale (PSS) (64). Sleep
quality was assessed via self-reports of how many days per week subjects
woke up feeling rested. This research was conducted under conditions of
informed consent with human subject clearance from the Institutional Re-
view Boards of the University of North Carolina at Chapel Hill and North-
western University.
Salivary T Collection and Measurement. The same saliva collection procedures
were used in 2005 and 2009. Each participant was provided with instructions
and two polypropylene tubes for saliva collection. The rst sample was
collected immediately before bed (PM) at mean sampling times of 10:14 PM ±
1:38 (SD) in 2005 and 10:04 PM ±1:33 (SD) in 2009. The participants were
instructed to collect the second sample immediately on waking the follow-
ing morning (AM) and to report the time of saliva collection. Mean AM
sampling times were 6:30 AM ±1:13 (SD) in 2005 and 6:48 AM ±1:28 (SD) in
2009. Saliva tubes were collected on the second day by an interviewer and
stored at 35 °C until shipment on dry ice to Northwestern University, where
they were stored at 80 °C.
Salivary T Assessment. T concentrations were determined at the Laboratory
for Human Biology Research at Northwestern University using an enzyme
immunoassay protocol developed for use with saliva samples (kit no. 1-2402;
Salimetrics). Interassay coefcients of variation were 13.7% and 11.5% for
high and low control samples, respectively, in 2005 samples and 7.8% and
17.9% for high and low control samples, respectively, in 2009 samples.
Sample Selection. During a 1-y period in 19831984, the CLHNS surveyed
w28,000 households in randomly selected neighborhoods in metro Cebu
City, inviting all pregnant women to participate (acceptance rate of 96%, n=
3,080 singleton liveborns). Thus, the original sample was representative of
births during that year. Subsequent attrition has largely been attributable to
out-migration, and the refusal rate for the subjects in adult surveys has
typically been w5% (61). During the 2005 survey, 1,008 (62%) of the original
cohort of 1,633 liveborn males were located and were willing to be inter-
viewed, and 908 (56% of original cohort) men were located and enrolled in
2009. Subjects lost to attrition have generally been from higher socioeco-
nomic status households (61), which is also true of the present sample (see
below). Participants were compensated 100 pesos (w$2 US) for their time.
Anal sample of 624 individuals had all required data and met all criteria
for inclusion. Seventy-three men were excluded from this analysis because
they were nightshift workers or had sleep patterns consistent with night-
shift work, which is associated with disrupted circadian rhythms for T (65,
66). Four subjects were excluded as outliers because their T values were very
high (all were 6+ SD above the sample mean), suggesting contamination of
the saliva sample by blood, and one subject was excluded because of a T
value below the assay detection limit. Because this sample is drawn from
a cultural setting in which it is rare for men to become new fathers outside
of stable romantic partnerships or to le for divorce, there were few single
new fathers (n=12)ordivorcedmen(n= 9), who therefore were excluded
from longitudinal analyses. We assessed whether subjects in the analysis
differed from excluded men. Excluded individuals were born to mothers
(P<0.10) and fathers (P<0.01) who were more educated. However, there
were no signicant differences between the subsample used here and
the original baseline cohort in birth weight, birth length, birth order,
household income, parental height, or mother or fathers age at baseline
(all P>0.10).
Statistical Analysis. All analyses were conducted using version 10 of Stata
(Stata Corporation). AM T (pg/mL), PM T (pg/mL), sleep quality, and PSS were
all analyzed as continuous variables. AM and PM T were each adjusted for
time of sampling (AM and PM) and usual wake time (AM) before calculating
absolute change in T (ΔT) between baseline (2005) and follow-up (2009). All
other models were also adjusted for time of sampling (AM and PM) and
usual wake time (AM). Average self-reported stress and sleep quality were
calculated as the mean of 2005 and 2009 values (26).
Paired ttests were used to compare adjusted values of baseline T and
follow-up T. Multiple logistic regression was used to predict 2009 partner-
ship and fatherhood status from baseline T (z-scored) among men who were
single or were not fathers in 2005. Multiple linear regression was used to
predict ΔT based on partnership and fatherhood status changes between
2005 and 2009, controlling for sleep quality and self-reported stress, among
men who were single nonfathers at baseline. Multiple linear regression was
used to predict ΔT among men who were single nonfathers at baseline,
based on the age of fathersyoungest child at follow-up, with child age
stratied according to whether the youngest child was a perinatal infant (1
mo old or less), nonperinatal infant (older than 1 mo but less than 1 y), or
noninfant (older than 1 y). Multiple linear regression was also used to assess
differences in fathersT values (2009) based on self-reported hours spent in
direct physical childcare. Ordered logistic regression was used to predict
mens self-reported hours spent in direct physical childcare (2009) from
baseline T (z-scored) among men who were single nonfathers in 2005, with
all models meeting the parallel regression assumption based on the Brant
test. Statistical signicance was evaluated at P<0.05, with relationships with
0.05 <P<0.10 interpreted as a borderline statistical trend. All regression
models were tested for heteroscedasticity using the BreuschPagan/Cook
Weisberg test and were calculated with robust SEs where appropriate.
ACKNOWLEDGMENTS. Linda Adair played a central role in designing and
implementing the CLHNS survey from which these data and samples were
obtained. Greg Duncan provided statistical advice. Jeffrey Huang helped
with laboratory work. We thank the Ofce of Population Studies, University
of San Carlos, Cebu, Philippines, for its role in study design and data
collection and the Filipino participants who provided their time for this
study. This work was supported by the Wenner Gren Foundation (Grants
7356 and 8186), National Science Foundation (Grants BCS-0542182 and BCS-
0962212), Interdisciplinary Obesity Center (Grant RR20649), and Center for
Environmental Health and Susceptibility (Grant ES10126; Project 7-2004-E).
L.T.G. was supported by a National Science Foundation Graduate Research
Fellowship during write-up.
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