Final answer:
The first test required to estimate heritability in twin studies is the Intraclass Correlation, as it assesses similarity between related individuals.
Step-by-step explanation:
The first graphical/statistical test required to estimate heritability (h2) via twin studies is the Intraclass Correlation. This type of correlation is used to assess the similarity between related individuals, which in the case of twin studies, refers to monozygotic or dizygotic twins. The Pearson Correlation Coefficient, while it measures the linear relationship between two variables, is not specifically designed for assessing the resemblance between groups of related individuals or sets of measurements taken in the same conditions, making Intraclass Correlation the appropriate choice among the options provided.