Experimental Design Details
The experiment will follow a 2x2 experimental design resulting in 4 experimental conditions: 1) Stress Task & Simple Shopping Choice, 2) Stress Task & Complex Shopping Choice, 3) Control Task & Simple Shopping Choice, 4) Control Task & Complex Shopping Choice.
These experimental conditions will be pre-assigned at the session level. This pre-assignment of sessions to experimental conditions ensures balance in terms of day of the week and time of day. When signing up for participation in the experiment, participants are asked to indicate their preferred session slots, but will not be informed in advance of the treatments associated with each time slot. If participants indicate availability for multiple slots, they are assigned to one of the slots solely based on scheduling concerns.
Upon arrival at our lab facilities, participants’ body weight and body height is measured without shoes and heavy clothing. Throughout the experimental session, participants are asked to wear an armband monitoring their heart rate using an optical sensor. At the beginning of the experimental session, participants are asked to provide a first saliva sample.
Following this, participants are asked to complete a 10 minute task. The nature of the task depends on the session’s randomly assigned experimental condition:
In conditions 1) and 2) (detailed above), i.e. the stress treatment groups, participants are asked to complete an incentivised task aimed at inducing mild stress. More specifically, subjects are asked to complete a 10 minute block of short incentivised decision tasks. While the tasks are completed individually, incentives are based on the joint performance of “social groups”, each consisting of 2 participants in the same session which are randomly matched , to elicit social stress. Incorrect answers and incomplete tasks are penalized. Time pressure is induced by the tight overall time limit as well as by time penalties and time limits per task. Short incentivised knowledge questions will appear on screen at seemingly random times throughout the course of the task block to induce additional stress through distractions. This stress protocol is designed to mimic stressors often experienced by low-socioeconomic mothers: making decisions with consequences for others (e.g. for the family) subject to financial and time constraints as well as distractions (e.g. by children requiring attention).
In conditions 3) and 4), i.e. the control groups, participants are asked to complete a task of similar nature but with no stress inducing features. Specifically, they are asked to answer 14 simple knowledge questions after reading 7 short texts about a variety of topics. The correct answers to each question can be found in the corresponding text. The questions are similar to those asked as distractions during the stress task. Subjects are given 10 minutes for this task, there are no consequences from not completing all questions. The task is not incentivised and no “social groups” are formed. Asking simple questions after providing the answers guarantees that participants will not experience stress due to task difficulty. Due to the lack of individual or group incentives, participants should not feel under time pressure or social stress. Furthermore, there are no interruptions creating stress.
Following the first task, participants are asked to complete a “food shopping” task. Participants are given 10 minutes to allocate a fixed budget of £30 to food and drink items offered in the “virtual supermarket” tool. A variety of low-calorie and high-calorie food and drink items is available to choose from with prices matching market prices at a local supermarket. In total, approximately 150 products are on offer. Participants are encouraged to make their shopping choices as they would during a weekly shop at their local supermarket. The supermarket choice is incentivised: 1 out of 15 participants are randomly chosen to receive their chosen basket delivered to their home approximately two weeks after the session. This incentive scheme was chosen to motivate participants to make choices representative of normal shopping behaviour.
Participants are informed that if they are selected and have not spent the entire £30 budget, they will be paid the difference in cash up to £2 maximum. This is to discourage non-representative shopping choices aimed at spending exactly £30, and to ensure that participants do not feel the pressure to spend the exact amount, which could induce stress for all participants. Under this incentive scheme it is optimal for participants to aim to spend between £28 and £30.
The complexity of the food shopping environment depends on the experimental condition assigned to the session: In conditions 1) and 3) (detailed above), i.e. the simple shopping choice, products are listed separately in 10 different food categories. In conditions 2) and 4), i.e. the complex shopping choice, products are shown in a single long list, grouped by category but without labelling of categories. In both choice environments approximately 150 items are on offer. The order in which items are displayed within each category is randomized at the participant level to avoid order effects. Furthermore, the display order of categories and the first category shown when opening the supermarket tool are randomized.
After the “food shopping” task, participants are asked to provide a second saliva sample (approx. 25-30 mins after start of stress / control task) and then given a five minute break. After the break, participants are asked to complete a questionnaire on demographics, family characteristics and behaviours which might impact cortisol levels. During the break and the time given to complete the first questionnaire, participants are given permission to consume the snacks provided on their desks: a bowl of high-calorie and a bowl of low-calorie snacks (not labelled as such or in any other way). After completion of the first questionnaire, the bowls of snacks are collected.
Participants are then asked to complete a second questionnaire. The questionnaire features questions about food consumption and food preferences of the participant and their youngest child as well as the participant’s food consumption during pregnancy. The questionnaire furthermore includes questions about the stressfulness of the stress/control task, chronic stress, participants’ coping behaviours when dealing with stress and about potentially stressful events during the last 3 months as well as during the pregnancy.
At the end of the experimental session, a final saliva sample is collected (approx. 80-90 mins after start of stress / control task).
Before receiving their payment, participants are debriefed: It is explained that the snacks provided differed in calorie content.