*Father trials: effects of a soft baby carrier on fathers* hormonal levels, neural processing of infant signals, and parenting behavior*

In most western countries, fathers have increased their participation in
parenting over the past decades. Even though in most families the participation
of fathers in child rearing is modest, the parental role of fathers is highly
relevant for child development (e.g., Kok et al., 2015; Ramchandani et al.,
2005). Nevertheless, fathers spend on average less than half the amount of time
in direct one-on-one interaction with their children as mothers (Huerta et al.,
2013), especially in early childhood (Wood & Repetti, 2004). Although the
quantity of time invested in parenting is generally considered less important
than the quality (*quality time*), it takes considerable time to get to know an
infant, become aware of its preferences, and read its signals (Bowlby, 1989).
For most young fathers, spending more time in physical contact or on
interaction with their infant may add to the quality of the interaction. As
such, spending time in close physical proximity of the child, for example using
a soft baby carrier in which the infant is chest to chest with his/her father,
may increase the quality of the father-infant relationship.
Here, we propose to conduct a randomized controlled trial (RCT) of infant
carrying with between-subject design in first time fathers. As parenting may be
influenced by hormonal levels and vice versa (with a special role for oxytocin,
vasopressin, cortisol, and testosterone) (Apter-Levi et al., 2014; Atzil et
al., 2012; Bick & Dozier, 2010; Feldman & Bakermans-Kranenburg, 2017), we will
test the effects of the use of a baby carrier on fathers* hormonal levels,
their processing of infant signals, and on their parenting behavior, including
the quantity (involvement) and quality (sensitivity) of father-child
interaction. A special focus is on a dimension of parenting that has received
considerable attention in animal research but, despite its evolutionary
importance, not in studies on humans: the role of the parent as protector.

Primary objectives
In a randomized control trial (RCT) the following hypotheses will be tested:
1. The behavioral intervention with a baby carrier in the early postnatal phase
results in different hormonal, neural and behavioral responses to infant
stimuli and video clips.
2. The baby carrier intervention promotes fathers* parenting in terms of
quantity (involvement) and quality (sensitivity).
3. The baby carrier intervention affects fathers* basal hormonal levels that
may mediate neural and behavioral effects.

Secondary objective
We will examine the extent to which effects of the baby carrier intervention
are moderated by fathers* early childhood experiences. Although fathers* early
childhood experiences are known to moderate effects of nasally administered
oxytocin (Bakermans-Kranenburg & Van IJzendoorn, 2013), it is not yet clear
whether this also holds for endogenously produced oxytocin.

We propose a RCT with a between-subject design in first time fathers. Fathers
will participate in a pretest when their child is 2 months of age. The
intervention starts 1-2 weeks after the pretest and lasts for three weeks. The
first posttest is administered one week after the intervention, and a follow-up
four months later.

This intervention aims at increasing the amount of physical contact between
father and child through the availability and instruction in the use of a soft
baby carrier. Fathers in the intervention group (half of the sample, randomly
selected) will receive an ergonomic soft baby carrier. Infants in the carrier
are chest to chest with their father, supported by the adult's upper torso.
Fathers are requested to use the baby carrier for at least six hours per week,
spread over a minimum of four days, for three weeks. Fidelity will be measured
using a motion logger and a temperature data logger fixed to the baby carrier.
In parallel with the intervention sessions, the fathers in the control group
are invited to have their baby close by in a baby seat, a Doomoo seat (Doomoo,
2004), inducing proximity between the father and baby without an increase of
physical contact. Fathers are asked to use the baby seat for at least six hours
per week, spread over a minimum of four days, for three weeks. Fathers
receiving the control intervention who use a baby carrier will not be
discouraged from doing so. We will use intent-to-treat analyses, keeping them
in the control group.
After the intervention, the fathers may keep the baby carrier (without the
loggers) or the baby seat until the follow-up. The use of the baby carrier or
baby seat during that time period will be evaluated via an app around two
months after the posttest.

There are no risks associated with the assessments used in this study. Both the
ergonomic baby carrier and doomoo seat are considered age-appropriate and safe
for father and infant. No adverse effects have been reported in
participants/patients undergoing MRI at the currently available field
strengths. The short sound fragment used in the Auditory Startling Task is not
harmful for the hearing of both father and infant. Once we understand the
neurobiological underpinnings of good-enough and poor parental sensitivity and
protection, better attempts can be made to improve parenting and reduce the
adverse effects of poor parenting. Thus, the importance of the benefits gained
from this research outweighs the minimal risks involved.