A sub-group of the families of twins was seen in the lab where extensive psycho-physiological, hormonal, and behavioral measures were taken. Behavior observations were conducted in a controlled setting and cortisol samples were collected. The other subgroup was only seen at home.

The laboratory assessments were followed within 2 weeks by a home visit to obtain exhaustive social, demographic, health, and further behavioral data on the twins and their families, mainly through interviews of both parents, self-report questionnaires filled out by both parents, and direct observation of the infant, home and neighborhood by the interviewer.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Families seen in the lab at 6 months were again seen in the lab and at home and families seen only at home at 6 months were seen only at home.

In the lab, extensive psycho-physiological, hormonal, and behavioral measures were taken. Behavior observations were conducted in a controlled setting and cortisol samples were collected.

The laboratory assessments were followed within 2 weeks by a home visit to obtain exhaustive social, demographic, health, and further behavioral data on the twins and their families, mainly through interviews of both parents, self-report questionnaires filled out by both parents, and direct observation of the infant, home and neighborhood by the interviewer.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

During the home visit, data was collected about social, demographic, health, and further behavioral characteristics of the twins and their families, mainly through interviews of both parents, self-report questionnaires filled out by both parents, and direct observation of the infant, home and neighborhood by the interviewer.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

During the home visit, data was collected about social, demographic, health, and further behavioral characteristics of the twins and their families, mainly through interviews of both parents, self-report questionnaires filled out by both parents, and direct observation of the infant, home and neighborhood by the interviewer.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics. Finally, direct and multi-faceted assessments of cognitive development were performed.

Data was collected during a lab visit in the summer for a battery of cognitive measures. Direct and multi-faceted assessments of cognitive development, with a strong emphasis on school readiness were performed. Cortisol samples were also collected.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Data was collected using school-based assessments including behavioral observations which were conducted in a controlled setting, cognitive assessments, and peer and teacher assessments. Direct and multi-faceted assessments of cognitive development, with a strong emphasis on school readiness were performed as well. In addition, peer relationships and experiences were measured through peers’ assessments and classroom sociometric interviews. Cortisol samples were also collected.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Data was collected using school-based assessments including behavioral observations which were conducted in a controlled setting, cognitive assessments, and peer and teacher assessments. Direct and multi-faceted assessments of cognitive development, with a strong emphasis on school readiness were performed as well. In addition, peer relationships and experiences were measured through peers’ assessments, classroom sociometric interviews and school achievement.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Information about health, parenting and behaviour was collected through questionnaires and semi-structured interviews.

Intelligence was assessed using the Wechsler Intelligence Scale for Children. Also, symptoms of inattention, impulsiveness, and hyperactivity were assessed through a battery of different neuropsychological tests, including a mixed go/no-go and stop task (measuring both restrain and cancellation in a single paradigm), continuous performance task (oddball), and a visuospatial working memory task.

Brain electrical activity was also recorded using a large array of electrodes (electroencephalography/event-related potential (EEG/ERP)) during these tests, and brain anatomical magnetic resonance imaging data was collected.

Blood and saliva samples were collected for the genotyping of parents and twins.

Data was collected from teacher ratings and children’s school achievement.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics. Finally, direct and multi-faceted assessments of cognitive development were performed.

Data was collected using school-based assessments including behavioral observations which were conducted in a controlled setting, cognitive assessments, and peer and teacher assessments. Direct and multi-faceted assessments of cognitive development were performed as well. In addition, peer relationships and experiences were measured through peers’ assessments, classroom sociometric interviews and school achievement.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Data was collected using school-based assessments including behavioral observations which were conducted in a controlled setting, cognitive assessments, and peer and teacher assessments. Direct and multi-faceted assessments of cognitive development were performed as well. In addition, peer relationships and experiences were measured through peers’ assessments, classroom sociometric interviews and school achievement.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Data was collected using school-based assessments including behavioral observations which were conducted in a controlled setting, cognitive assessments, and peer and teacher assessments. Direct and multi-faceted assessments of cognitive development were performed as well. In addition, peer relationships and experiences were measured through peers’ assessments, classroom sociometric interviews and school achievement. In secondary school, the self-ratings also covered substance use, gambling, peer relationships, and psychological adjustment.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Data was collected using school-based assessments including behavioral observations which were conducted in a controlled setting, cognitive assessments, and peer and teacher assessments. Direct and multi-faceted assessments of cognitive development were performed as well. In addition, peer relationships and experiences were measured through peers’ assessments, classroom sociometric interviews and school achievement. In secondary school, the self-ratings also covered substance use, gambling, peer relationships, and psychological adjustment. Cortisol samples were also collected.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Data was collected using school-based assessments including behavioral observations which were conducted in a controlled setting, cognitive assessments, and peer and teacher assessments. Direct and multi-faceted assessments of cognitive development were performed as well. In addition, peer relationships and experiences were measured through peers’ assessments, classroom sociometric interviews and school achievement. In secondary school, the self-ratings also covered substance use, gambling, peer relationships, and psychological adjustment.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Data was collected using school-based assessments including behavioral observations which were conducted in a controlled setting, cognitive assessments, and peer and teacher assessments. Direct and multi-faceted assessments of cognitive development were performed as well. In addition, peer relationships and experiences were measured through peers’ assessments, classroom sociometric interviews and school achievement. In secondary school, the self-ratings also covered substance use, gambling, peer relationships, and psychological adjustment.

Parenting behaviors were assessed yearly, through mother and father self-reports of parental warmth, self-efficacy and impact, overprotection, and hostile-reactive behavior. Social behaviors are also assessed yearly using a variant of the Child Social Behaviour Questionnaire. Also, some of the measures of the children’s environment collected regularly were about nutrition, the mental health status of parents, marital satisfaction and status, as well as a variety of socio-economic and demographic characteristics.

Twins’ self-efficacy beliefs, behavioral, social, psychological and academic/work-related adjustment, substance use and gambling habits as well as their perceived social support were assessed through an online questionnaire.

Blood spots, saliva, buccal cells and hair were collected along with data on their general health, sleeping habits and perception of pain.