Explain the concept of internal validity and give an example of a threat to it in developmental research.

• A. Internal validity is the generalizability of results across populations.
• B. Internal validity is the accuracy of measurement tools across trials.
• C. Internal validity is the degree to which observed effects are due to the manipulated variable; threats include confounds, maturation, and selection bias
• D. Internal validity is about the speed of data collection

Answer: (C)

Internal validity is about whether the observed effects in a study can be attributed to the experimental manipulation rather than to other factors. In developmental research, this matters because children change a lot over short periods, so you have to be sure that any improvement or difference you see is really due to what you did in the study, not just natural growth or other influences.

For example, if you run a program intended to boost early literacy and you compare participants who receive the program to those who don’t, maturation can threaten internal validity. If the children in the treatment group are tested after a longer period, they might show gains simply because kids tend to develop literacy skills as they age, not because the program caused the improvement. Without a proper control group or random assignment to separate the effects of time from the program, you can’t confidently claim the program created the change.

Other common threats include confounds, where another variable (like differences in teacher quality or classroom environment) unintentionally varies with the treatment and actually drives the outcome, and selection bias, where groups differ at the start in ways that affect results.

In contrast, generalizability across populations is about external validity, measurement tool accuracy across trials relates to measurement validity or reliability, and the speed of data collection isn’t a validity issue.