Because the cells of various tissues are exposed to hundreds of small molecules (metabolites), hormones and enzymes circulating through the body, the internal environment of an individual is perhaps best characterized by the (chemical) content of his/her blood. Most of those molecules are able to cross the blood–brain barrier, so they also bathe the brain cells. (we will describe tools for measuring the levels of such molecules [metabolomics and proteomics] in Chap. 6 and touch on the issues of their dependence/independence on the individual’s behaviour in Sect. 3.3 of this chapter.)

A major (external) source of small molecules is the food we eat. A number of instruments have been designed to capture the consumption frequency of a wide range of foods and drinks. In the ALSPAC Food Frequency Questionnaire, for example, informants are asked to indicate how often a given food/drink is “typically” consumed (Rogers and Emmett 1998; Northstone et al. 2011). A 24-hour food-recall samples in more detail all food consumed over the previous 24 h through a structured interview of the individual by a dietician; this information is then analysed using a recipe file to estimate the intake of macronutrients and micronutrients (e.g. Slimani et al. 2007).

The “body” environment represents the body’s interface between internal and external environments and consists of the epithelial surfaces lining the gastrointestinal, urinary and respiratory tracts and the epidermal surface of our skin. These surfaces contain a kind of auxiliary biological system called the microbiome, which coexists with each human being (Text Box 3.1.). This intimate living arrangement with trillions of other organisms has profound effects on the internal environment and therefore on human health. For example, microbial flora of the gut exert(s) a considerable influence on the absorption of nutrients, such as vitamins and essential fatty acids (James and Garza 2012); in turn, nutrients affect multiple biological processes occurring every day in the brain—from the synthesis of neurotransmitters, through the reshaping of dendrites, to the myelination of axons.

The social environment reflects the complexity of who we are as social beings. Its health implications are indisputable. It is a well-known fact that social isolation predicts morbidity and mortality (House et al. 1988; Holt-Lunstad et al. 2010). Put simply, “humans need others to survive and prosper” (Cacioppo et al. 2011). Expanding circles of the social space includes the individual’s family, workplace, neighbourhood, community and culture. Without a doubt, these social environments play a key role in shaping our brains and behaviour. But as discussed in Sect. 3.3, social environments in general, and the family environment in particular, are often confounded by our genes. We need to keep this in mind when using various measures of social environment as “exposures” when, in fact, they might represent “outcomes”.

When reconstructing a child’s family environment, we need to focus on the primary caregivers, their circumstances and their behaviour. To start with, socio-economic circumstances in childhood can foreshadow adult health (Text Box 3.2.). It is relatively easy to acquire information about basic socio-demographic characteristics of the child’s family—indeed, a number of instruments exist that serve this purpose well (Table 3.1).

Maternal care is a powerful modulator of the brain systems important for stress responsivity (reviewed in Champagne and Curley 2009). Therefore, it is vital to examine caregiving-related issues—from ascertaining the primary caregiver to evaluating family relationships in infancy and childhood. A number of instruments have been designed and validated for this purpose (Table 3.1). In general, information collected with such instruments should be corroborated by multiple informants whenever possible.

During adolescence, the influence of peers and the school environment begins to combine with that of the nuclear family. A number of instruments are suitable for capturing the nature of social experiences in this period of life. For example, Lerner and colleagues developed a questionnaire called Positive Youth Development (~300 items), based on the “5 Cs” model: competence, confidence, character, social connection and caring. The questionnaire inquires about “contextual assets” available to the adolescent vis-à-vis his/her friends, community, family and school climate, as well as his/her participation in extra-curricular activities (Lerner et al. 2005).

In adult life, “exposures” associated with the workplace environment represent a powerful cumulative factor shaping our brains, eight hours a day, five days a week, for 40+ years. The brains of taxi drivers (Maguire et al. 2000), musicians (Gaser and Schlaug 2003) or typists (Scheibel et al. 1990) differ from the “average brain” in certain aspects relevant to their profession, thus pointing to experience-driven plasticity as one of the sources of inter-individual variability in brain structure and function. But the workplace is also the source of other “exposures” related to social interactions, the most common one being stress related to job demands and social hierarchies (Text Box 3.3.). Thus, in addition to capturing the individual’s occupation, employment history and income, we should also attempt to characterize the nature of interpersonal interactions at the workplace, as well as job satisfaction and perceived stress. For adults as well as children, family is an important source of social interactions: we should record marital status, number of children and the physical and psychological closeness of an extended family, as well as the frequency and context of family interactions (e.g. common meals).

Finally, the community and culture in which the person lives represent—from a social perspective—a potential source of both positive and negative “exposures”; by definition, these depend on the nature of social interactions between the individual and other people in his/her physical (or virtual) community. Here, broader social networks available to the individual can be assessed using a variety of instruments. Finally, cultural identity should be carefully examined, especially in population studies based in multicultural settings, where immigration-related factors provide powerful sources of inter-individual variations Text Box 3.4.