Present Perfect Simple
"International studies of personality have shown..."
- Justification: The present perfect simple is used here to indicate that the action of showing the results of international studies of personality is relevant to the present moment. It implies that these studies have been conducted in the past, and their relevance continues up to the present time.
"A recent series of studies also noted that islanders are less talkative..."
- Justification: The present perfect simple is used to describe a series of studies that have been completed recently and are still relevant to the present discussion about the personality traits of islanders.
Past Simple
"When psychologists gave the same personality test to large groups of people from different nations a few years ago..."
- Justification: The past simple is used here to describe a specific action that occurred in the past—psychologists gave the personality test to large groups of people. The time frame is clearly defined as "a few years ago," indicating a completed action.
"Another huge study looked at similarities in average personality across broader geographical areas..."
- Justification: The past simple is used to describe the action of another study that took place in the past, examining similarities in average personality across geographical areas. The past tense indicates that this action has been completed.